Method for performing measurement and terminal

ABSTRACT

A method is provided for performing a measurement. The method may be performed by a user equipment (UE) and includes applying both of a measurement subframe pattern for a neighbor cell and a measurement timing configuration for a discovery signal, and selecting at least one or more subframes, to perform the measurement according to the appliance of the measurement subframe pattern and the measurement timing configuration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 15/496,787 filed on Apr. 25, 2017, which is a Continuation ofU.S. patent application Ser. No. 14/694,536 filed on Apr. 23, 2015 (nowU.S. Pat. No. 9,668,152 issued on May 30, 2017), which claims prioritybenefit under 35 U.S.C. § 119(e) to U.S. Provisional Application Nos.61/984,017 filed on Apr. 24, 2014, 61/989,518 filed on May 6, 2014,61/993,286 filed on May 15, 2014, 62/009,866 filed on Jun. 9, 2014 and62/034,797 filed on Aug. 8, 2014, all of which are hereby expresslyincorporated by reference into the present application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to mobile communications.

Discussion of the Related Art

3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) thatis an advancement of UMTS (Universal Mobile Telecommunication System) isbeing introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonalfrequency division multiple access) is used for downlink, and SC-FDMA(single carrier-frequency division multiple access) is used for uplink.To understand OFDMA, OFDM should be known. OFDM may attenuateinter-symbol interference with low complexity and is in use. OFDMconverts data serially input into N parallel data pieces and carries thedata pieces over N orthogonal sub-carriers. The sub-carriers maintainorthogonality in view of frequency. Meanwhile, OFDMA refers to amultiple access scheme that realizes multiple access by independentlyproviding each user with some of sub-carriers available in the systemthat adopts OFDM as its modulation scheme.

As set forth in 3GPP TS 36.211 V10.4.0, the physical channels in 3GPPLTE may be classified into data channels such as PDSCH (physicaldownlink shared channel) and PUSCH (physical uplink shared channel) andcontrol channels such as PDCCH (physical downlink control channel),PCFICH (physical control format indicator channel), PHICH (physicalhybrid-ARQ indicator channel) and PUCCH (physical uplink controlchannel).

In a next-generation mobile communication system, it is expected that asmall cell having a small cell coverage radius will be added to thecoverage of an existing cell and a small cell will process more traffic.

However, if small cells within the coverage of a macro cell are denselydeployed, it may be difficult for UE to detect the small cells within ashort time.

SUMMARY OF THE INVENTION

Accordingly, the disclosure of this specification is to solve theaforementioned problem.

In order to achieve the object, the disclosure of this specification isto provide a method for performing measurements. The method may beperformed by a user equipment (UE) and comprise: receiving a measurementsubframe pattern for a neighbor cell and a measurement timingconfiguration for a discovery signal; selecting at least one or moresubframes to perform the measurement based on both of the measurementsubframe pattern and the measurement timing configuration; andperforming the measurement by using the discovery signal of the neighborcell on the selected subframes.

The subframes on which the measurement may be performed correspond to atleast one or more overlapped subframes between the measurement subframepattern and the measurement timing configuration.

The selecting step may include: selecting specific subframes based onthe measurement subframe pattern; and selecting the at least one or moresubframes among the specific subframes based on the measurement timingconfiguration.

The measurement timing configuration may be configured per carrierfrequency.

The method may further comprise: if the neighbor cell is in thedeactivated state, using the discovery signal rather than acell-specific reference signal (CRS) to perform the measurements.

The discovery signal may be a signal based on at least one ofcell-specific reference signal (CRS), a channel-state informationreference signal (CSI-RS), a primary synchronization signal (PSS) and asecondary synchronization signal (SSS).

If the measurement is for measuring a received signal strength indicator(RSSI), the measurement may be performed on entire OFDM symbols of asubframe.

In order to achieve the object, the disclosure of this specification isto provide a user equipment (UE) for performing measurements. The UE maycomprise: a radio frequency (RF) unit configured to receive ameasurement subframe pattern for a neighbor cell and a measurementtiming configuration for a discovery signal; and a processor configuredto select at least one or more subframes to perform the measurementbased on both of the measurement subframe pattern and the measurementtiming configuration and perform the measurement by using the discoverysignal of the neighbor cell on the selected subframes.

In accordance with the disclosure of this specification, theaforementioned conventional problem is solved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication system.

FIG. 2 illustrates the structure of a radio frame according to FDD in3GPP LTE.

FIG. 3 illustrates the structure of a downlink radio frame according toTDD in 3GPP LTE.

FIG. 4 is an exemplary diagram illustrating a resource grid for a singleuplink or downlink slot in 3GPP LTE.

FIG. 5 illustrates the structure of a downlink subframe.

FIG. 6 illustrates the structure of an uplink subframe in 3GPP LTE.

FIG. 7 illustrates a frame structure for the transmission of asynchronization signal in an FDD frame.

FIG. 8 illustrates an example of a frame structure for sending asynchronization signal in a TDD frame.

FIG. 9 illustrates an example of a pattern in which CRSs are mapped toRBs if an eNodeB uses a single antenna port.

FIG. 10 illustrates measurement and measurement report procedures.

FIG. 11 illustrates an example of RBs to which CSI-RSs are mapped inreference signals.

FIG. 12 is a diagram illustrating a heterogeneous network environment inwhich a macro cell and small cells having a possibility that they maybecome a next-generation wireless communication system are mixed.

FIG. 13 is an exemplary diagram of eICIC (enhanced Inter-CellInterference Coordination) for solving interference between eNodeBs.

FIG. 14 is an exemplary diagram illustrating the situation in whichsmall cells have been densely deployed.

FIG. 15 illustrates an example in which small cells send discoverysignals according to the disclosure of this specification.

FIG. 16 illustrates an example in which a plurality of transmissionpoints (TPs) (or small cells) within a cluster uses the same physicalcell identifier (PCID).

FIG. 17a is an exemplary diagram of a first solution regarding thatmeasurement will be performed using which one of a CRS and a DS.

FIG. 17b is a more detailed exemplary diagram of the first solutionregarding that measurement will be performed using which one of a CRSand a DS.

FIG. 18 illustrates a process of determining a subframe on which UE willperform measurement if both a measurement subframe pattern and a DMTCare used.

FIGS. 19a and 19b illustrate examples in which a subframe on whichmeasurement is to be performed based on both a measurement subframepattern and a DMTC.

FIG. 20 illustrates another example in which transmission timing ofdiscovery signals is different between cells.

FIG. 21 is a block diagram illustrating a wireless communication systemin which the disclosure of this specification is implemented.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, based on 3rd Generation Partnership Project (3GPP) longterm evolution (LTE) or 3GPP LTE-advanced (LTE-A), the present inventionwill be applied. This is just an example, and the present invention maybe applied to various wireless communication systems. Hereinafter, LTEincludes LTE and/or LTE-A.

The technical terms used herein are used to merely describe specificembodiments and should not be construed as limiting the presentinvention. Further, the technical terms used herein should be, unlessdefined otherwise, interpreted as having meanings generally understoodby those skilled in the art but not too broadly or too narrowly.Further, the technical terms used herein, which are determined not toexactly represent the spirit of the invention, should be replaced by orunderstood by such technical terms as being able to be exactlyunderstood by those skilled in the art. Further, the general terms usedherein should be interpreted in the context as defined in thedictionary, but not in an excessively narrowed manner.

The expression of the singular number in the specification includes themeaning of the plural number unless the meaning of the singular numberis definitely different from that of the plural number in the context.In the following description, the term ‘include’ or ‘have’ may representthe existence of a feature, a number, a step, an operation, a component,a part or the combination thereof described in the specification, andmay not exclude the existence or addition of another feature, anothernumber, another step, another operation, another component, another partor the combination thereof.

The terms ‘first’ and ‘second’ are used for the purpose of explanationabout various components, and the components are not limited to theterms ‘first’ and ‘second’. The terms ‘first’ and ‘second’ are only usedto distinguish one component from another component. For example, afirst component may be named as a second component without deviatingfrom the scope of the present invention.

It will be understood that when an element or layer is referred to asbeing “connected to” or “coupled to” another element or layer, it can bedirectly connected or coupled to the other element or layer orintervening elements or layers may be present. In contrast, when anelement is referred to as being “directly connected to” or “directlycoupled to” another element or layer, there are no intervening elementsor layers present.

Hereinafter, exemplary embodiments of the present invention will bedescribed in greater detail with reference to the accompanying drawings.In describing the present invention, for ease of understanding, the samereference numerals are used to denote the same components throughout thedrawings, and repetitive description on the same components will beomitted. Detailed description on well-known arts which are determined tomake the gist of the invention unclear will be omitted. The accompanyingdrawings are provided to merely make the spirit of the invention readilyunderstood, but not should be intended to be limiting of the invention.It should be understood that the spirit of the invention may be expandedto its modifications, replacements or equivalents in addition to what isshown in the drawings.

As used herein, ‘base station’ generally refers to a fixed station thatcommunicates with a wireless device and may be denoted by other termssuch as eNB (evolved-NodeB), BTS (base transceiver system), or accesspoint.

Furthermore, UE (user equipment) used herein may be fixed or may havemobility and may be called another term, such as a device, a wirelessdevice, a terminal, an MS (mobile station), a UT (user terminal), an SS(subscriber station, or an MT (mobile terminal).

As used herein, ‘user equipment (UE)’ may be stationary or mobile, andmay be denoted by other terms such as device, wireless device, terminal,MS (mobile station), UT (user terminal), SS (subscriber station), MT(mobile terminal) or other name.

FIG. 1 shows a wireless communication system.

The wireless communication system includes at least one base station(BS) 20. Respective BSs 20 provide a communication service to particulargeographical areas 20 a, 20 b, and 20 c (which are generally calledcells). Each cell may be divided into a plurality of areas (which arecalled sectors).

The UE generally belongs to one cell and the cell to which the UEbelongs is referred to as a serving cell. A base station that providesthe communication service to the serving cell is referred to as aserving BS. Since the wireless communication system is a cellularsystem, another cell that neighbors to the serving cell is present.Another cell which neighbors to the serving cell is referred to aneighbor cell. A base station that provides the communication service tothe neighbor cell is referred to as a neighbor BS. The serving cell andthe neighbor cell are relatively decided based on the UE.

Hereinafter, a downlink means communication from the base station 20 tothe UE 10 and an uplink means communication from the UE 10 to the basestation 20. In the downlink, a transmitter may be a part of the basestation 20 and a receiver may be a part of the UE 10. In the uplink, thetransmitter may be a part of the UE 10 and the receiver may be a part ofthe base station 20.

Meanwhile, the wireless communication system may be generally dividedinto a frequency division duplex (FDD) type and a time division duplex(TDD) type. According to the FDD type, uplink transmission and downlinktransmission are achieved while occupying different frequency bands.According to the TDD type, the uplink transmission and the downlinktransmission are achieved at different time while occupying the samefrequency band. A channel response of the TDD type is substantiallyreciprocal. This means that a downlink channel response and an uplinkchannel response are approximately the same as each other in a givenfrequency area. Accordingly, in the TDD based wireless communicationsystem, the downlink channel response may be acquired from the uplinkchannel response. In the TDD type, since an entire frequency band istime-divided in the uplink transmission and the downlink transmission,the downlink transmission by the base station and the uplinktransmission by the terminal may not be performed simultaneously. In theTDD system in which the uplink transmission and the downlinktransmission are divided by the unit of a subframe, the uplinktransmission and the downlink transmission are performed in differentsubframes.

Hereinafter, the LTE system will be described in detail.

FIG. 2 shows a downlink radio frame structure according to FDD of 3rdgeneration partnership project (3GPP) long term evolution (LTE).

For the radio frame shown in FIG. 2, 3GPP (3rd Generation PartnershipProject) TS 36.211 V10.4.0 (2011-12) “Technical Specification GroupRadio Access Network; Evolved Universal Terrestrial Radio Access(E-UTRA); Physical channels and modulation (Release 8)”, Ch. 5 may bereferenced.

Referring to FIG. 2, a radio frame includes 10 sub-frames, and onesub-frame includes two slots. The slots in the radio frame are markedwith slot numbers 0 through 19. The time taken for one sub-frame to betransmitted is referred to as a TTI (transmission time duration). TheTTI may be the unit of scheduling for data transmission. For example,the length of one radio frame may be 10 ms, the length of one sub-framemay be 1 ms, and the length of one slot may be 0.5 ms.

The structure of a radio frame is merely an example, and the number ofsub-frames included in the radio frame or the number of slots includedin a sub-frame may vary differently.

Meanwhile, one slot may include a plurality of OFDM symbols. The numberof OFDM symbols included in one slot may vary depending on a cyclicprefix (CP). In particular, in 3GPP LTE, it is defined such that 7 OFDMsymbols are included in one slot in a normal CP case, and 6 OFDM symbolsare included in one slot in an extended CP case. OFDM symbol is merelyto represent one symbol period in the time domain since 3GPP LTE adoptsOFDMA (orthogonal frequency division multiple access) for downlink (DL),and the multiple access scheme or name is not limited thereto. Forexample, the OFDM symbol may be referred to as SC-FDMA (singlecarrier-frequency division multiple access) symbol or symbol period.

FIG. 3 illustrates the architecture of a downlink radio frame accordingto TDD in 3GPP LTE.

3GPP (3rd Generation Partnership Project) TS 36.211 V10.4.0 (2011-12)“Technical Specification Group Radio Access Network; Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical channels and modulation(Release 10)”, Ch. 4 may be referenced.

Sub-frames having index #1 and index #6 are denoted special sub-frames,and include a DwPTS (Downlink Pilot Time Slot: DwPTS), a GP (GuardPeriod) and an UpPTS (Uplink Pilot Time Slot). The DwPTS is used forinitial cell search, synchronization, or channel estimation in aterminal. The UpPTS is used for channel estimation in the base stationand for establishing uplink transmission sync of the terminal. The GP isa period for removing interference that arises on uplink due to amulti-path delay of a downlink signal between uplink and downlink.

In TDD, a DL (downlink) sub-frame and a UL (Uplink) co-exist in oneradio frame. Table 1 shows an example of configuration of a radio frame.

TABLE 1 UL-DL Switch-point Subframe index Configuration periodicity 0 12 3 4 5 6 7 8 9 0  5 ms D S U U U D S U U U 1  5 ms D S U U D D S U U D2  5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms D S U UD D D D D D 5 10 ms D S U D D D D D D D 6  5 ms D S U U U D S U U D

‘D’ denotes a DL sub-frame, ‘U’ a UL sub-frame, and ‘S’ a specialsub-frame. When receiving a UL-DL configuration from the base station,the terminal may be aware of whether a sub-frame is a DL sub-frame or aUL sub-frame according to the configuration of the radio frame.

TABLE 2 Normal CP in downlink Extended CP in downlink Special UpPTSUpPTS subframe Normal CP Extended CP Normal CP Extended CP configurationDwPTS in uplink in uplink DwPTS in uplink in uplink 0  6592*Ts 2192*Ts2560*Ts  7680*Ts 2192*Ts 2560*Ts 1 19760*Ts 20480*Ts 2 21952*Ts 23040*Ts3 24144*Ts 25600*Ts 4 26336*Ts  7680*Ts 4384*Ts 5120*ts 5  6592*Ts4384*Ts 5120*ts 20480*Ts 6 19760*Ts 23040*Ts 7 21952*Ts — 8 24144*Ts —

FIG. 4 illustrates an example resource grid for one uplink or downlinkslot in 3GPP LTE.

Referring to FIG. 4, the uplink slot includes a plurality of OFDM(orthogonal frequency division multiplexing) symbols in the time domainand NRB resource blocks (RBs) in the frequency domain. For example, inthe LTE system, the number of resource blocks (RBs), i.e., NRB, may beone from 6 to 110.

The resource block is a unit of resource allocation and includes aplurality of sub-carriers in the frequency domain. For example, if oneslot includes seven OFDM symbols in the time domain and the resourceblock includes 12 sub-carriers in the frequency domain, one resourceblock may include 7×12 resource elements (REs).

Meanwhile, the number of sub-carriers in one OFDM symbol may be one of128, 256, 512, 1024, 1536, and 2048.

In 3GPP LTE, the resource grid for one uplink slot shown in FIG. 4 mayalso apply to the resource grid for the downlink slot.

FIG. 5 illustrates the architecture of a downlink sub-frame.

In FIG. 5, assuming the normal CP, one slot includes seven OFDM symbols,by way of example.

The DL (downlink) sub-frame is split into a control region and a dataregion in the time domain. The control region includes up to first threeOFDM symbols in the first slot of the sub-frame. However, the number ofOFDM symbols included in the control region may be changed. A PDCCH(physical downlink control channel) and other control channels areassigned to the control region, and a PDSCH is assigned to the dataregion.

The physical channels in 3GPP LTE may be classified into data channelssuch as PDSCH (physical downlink shared channel) and PUSCH (physicaluplink shared channel) and control channels such as PDCCH (physicaldownlink control channel), PCFICH (physical control format indicatorchannel), PHICH (physical hybrid-ARQ indicator channel) and PUCCH(physical uplink control channel).

The PCFICH transmitted in the first OFDM symbol of the sub-frame carriesCIF (control format indicator) regarding the number (i.e., size of thecontrol region) of OFDM symbols used for transmission of controlchannels in the sub-frame. The wireless device first receives the CIF onthe PCFICH and then monitors the PDCCH.

Unlike the PDCCH, the PCFICH is transmitted through a fixed PCFICHresource in the sub-frame without using blind decoding.

The PHICH carries an ACK (positive-acknowledgement)/NACK(negative-acknowledgement) signal for a UL HARQ (hybrid automatic repeatrequest). The ACK/NACK signal for UL (uplink) data on the PUSCHtransmitted by the wireless device is sent on the PHICH.

The PBCH (physical broadcast channel) is transmitted in the first fourOFDM symbols in the second slot of the first sub-frame of the radioframe. The PBCH carries system information necessary for the wirelessdevice to communicate with the base station, and the system informationtransmitted through the PBCH is denoted MIB (master information block).In comparison, system information transmitted on the PDSCH indicated bythe PDCCH is denoted SIB (system information block).

The PDCCH may carry activation of VoIP (voice over internet protocol)and a set of transmission power control commands for individual UEs insome UE group, resource allocation of an upper layer control messagesuch as a random access response transmitted on the PDSCH, systeminformation on DL-SCH, paging information on PCH, resource allocationinformation of UL-SCH (uplink shared channel), and resource allocationand transmission format of DL-SCH (downlink-shared channel). A pluralityof PDCCHs may be sent in the control region, and the terminal maymonitor the plurality of PDCCHs. The PDCCH is transmitted on one CCE(control channel element) or aggregation of some consecutive CCEs. TheCCE is a logical allocation unit used for providing a coding rate perradio channel's state to the PDCCH. The CCE corresponds to a pluralityof resource element groups. Depending on the relationship between thenumber of CCEs and coding rates provided by the CCEs, the format of thePDCCH and the possible number of PDCCHs are determined.

The PBCH (physical broadcast channel) is transmitted in the first fourOFDM symbols in the second slot of the first sub-frame of the radioframe. The PBCH carries system information necessary for the wirelessdevice to communicate with the base station, and the system informationtransmitted through the PBCH is denoted MIB (master information block).In comparison, system information transmitted on the PDSCH indicated bythe PDCCH is denoted SIB (system information block).

The PDCCH may carry activation of VoIP (voice over internet protocol)and a set of transmission power control commands for individual UEs insome UE group, resource allocation of an upper layer control messagesuch as a random access response transmitted on the PDSCH, systeminformation on DL-SCH, paging information on PCH, resource allocationinformation of UL-SCH (uplink shared channel), and resource allocationand transmission format of DL-SCH (downlink-shared channel). A pluralityof PDCCHs may be sent in the control region, and the terminal maymonitor the plurality of PDCCHs. The PDCCH is transmitted on one CCE(control channel element) or aggregation of some consecutive CCEs. TheCCE is a logical allocation unit used for providing a coding rate perradio channel's state to the PDCCH. The CCE corresponds to a pluralityof resource element groups. Depending on the relationship between thenumber of CCEs and coding rates provided by the CCEs, the format of thePDCCH and the possible number of PDCCHs are determined.

In 3GPP LTE, blind decoding is used for detecting a PDCCH. The blinddecoding is a scheme of identifying whether a PDCCH is its own controlchannel by demasking a desired identifier to the CRC (cyclic redundancycheck) of a received PDCCH (this is referred to as candidate PDCCH) andchecking a CRC error. The base station determines a PDCCH formataccording to the DCI to be sent to the wireless device, then adds a CRCto the DCI, and masks a unique identifier (this is referred to as RNTI(radio network temporary identifier) to the CRC depending on the owneror purpose of the PDCCH.

When UE monitors a PDCCH based on a C-RNTI, a DCI format and a searchspace to be monitored are determined depending on transmission mode of aPDSCH. The following table shows an example of the monitoring of a PDCCHin which a C-RNTI is set.

TABLE 3 TRANSMISSION Transmission mode of PDSCH MODE DCI format Searchspace according to PDCCH Mode 1 DCI format 1A common and Single antennaport, port 0 UE-specific DCI format 1 UE-specific Single antenna port,port 0 Mode 2 DCI format 1A common and Transmit diversity UE-specificDCI format 1 UE-specific Transmit diversity Mode 3 DCI format 1A commonand Transmit diversity UE-specific DCI format 2A UE-specific CyclicDelay Diversity (CDD) or transmit diversity Mode 4 DCI format 1A commonand Transmit diversity UE-specific DCI format 2 UE-specific Closed-loopspatial multiplexing Mode 5 DCI format 1A common and Transmit diversityUE-specific DCI format 1D UE-specific Multi-user Multiple Input MultipleOutput (MU-MIMO) Mode 6 DCI format 1A common and Transmit diversityUE-specific DCI format 1B UE-specific Closed-loop spatial multiplexingMode 7 DCI format 1A common and If the number of PBCH UE-specifictransmission ports is 1, single antenna port, port 0, and if not,transmit diversity DCI format 1 UE-specific a single antenna port, port5 Mode 8 DCI format 1A common and If the number of PBCH UE-specifictransmission ports is 1, single antenna port, port 0, and if not,transmit diversity DCI format 2B UE-specific Dual layer transmission(port 7 or 8), or a single antenna port, port 7 or 8 Mode 9 DCI format1A common and Non-MBSFN subframe: If the UE-specific number of PBCHantenna ports is one, Single-antenna port, port 0 is used, otherwiseTransmit diversity. MBSFN subframe: Single-antenna port, port 7 DCIformat 2C UE-specific Up to 8 layer transmission, ports 7-14

Purposes of DCI formats are classified as follows.

TABLE 4 DCI format Contents DCI format 0 It is used for PUSCHscheduling. DCI format 1 It is used for scheduling of one PDSCHcodeword. DCI format 1A It is used for compact scheduling and randomaccess process of one PDSCH codeword. DCI format 1B It is used in simplescheduling of one PDSCH codeword having precoding information. DCIformat 1C It is used for very compact scheduling of one PDSCH codeword.DCI format 1D It is used for simple scheduling of one PDSCH codewordhaving precoding and power offset information. DCI format 2 It is usedfor PDSCH scheduling of UEs configured to a closed-loop spatialmultiplexing mode. DCI format 2A It is used for PDSCH scheduling of UEsconfigured to an open-loop spatial multiplexing mode. DCI format 3 It isused for transmission of a TPC command of a PUCCH and a PUSCH having a2-bit power adjustment. DCI format 3A It is used for transmission of aTPC command of a PUCCH and a PUSCH having a 1-bit power adjustment. DCIformat 4 It is used for PUSCH scheduling in one UL cell in amulti-antenna Tx mode.

FIG. 6 illustrates the architecture of an uplink sub-frame in 3GPP LTE.

Referring to FIG. 6, the uplink sub-frame may be separated into acontrol region and a data region in the frequency domain. The controlregion is assigned a PUCCH (physical uplink control channel) fortransmission of uplink control information. The data region is assigneda PUSCH (physical uplink shared channel) for transmission of data (insome cases, control information may also be transmitted).

A resource block pair (RB pair) is allocated to a PUCCH for one UE in asubframe. Resource blocks belonging to a resource block pair occupydifferent subcarriers in a first slot and a second slot. A frequencyoccupied by resource blocks belonging to a resource block pair allocatedto a PUCCH is changed based on a slot boundary. This is said that the RBpair allocated to the PUCCH has been frequency-hopped in the slotboundary.

The PUCCH for one terminal is assigned in resource block (RB) pair inthe sub-frame. The resource blocks in the resource block pair take updifferent sub-carriers in each of the first and second slots. Thefrequency occupied by the resource blocks in the resource block pairassigned to the PUCCH is varied with respect to a slot boundary. This isreferred to as the RB pair assigned to the PUCCH having beenfrequency-hopped at the slot boundary.

Since the UE transmits the uplink control information on a time basisthrough different subcarriers, a frequency diversity gain can beobtained. m is a location index indicating a logical frequency domainlocation of a RB pair allocated to a PUCCH in a subframe.

Examples of the uplink control information transmitted on a PUCCHinclude hybrid automatic repeat request (HARQ), acknowledgement(ACK)/non-acknowledgement (NACK), channel state information (CSI)indicating a DL channel state, scheduling request (SR) which is a ULradio resource allocation request, etc.

The PUSCH is mapped to an uplink shared channel (UL-SCH) which is atransport channel. Uplink data transmitted through the PUSCH may be atransport block which is a data block for the UL-SCH transmitted duringa TTI. The transport block may be user information. In addition, theuplink data may be multiplexed data. The multiplexed data may beobtained by multiplexing the control information and a transport blockfor the UL-SCH. Examples of the control information to be multiplexedwith data may include a channel quality indicator (CQI), a precodingmatrix indicator (PMI), HARQ, a rank indicator (RI), etc. Alternatively,the uplink data may consist of only the control information.

<Carrier Aggregation (CA)>

A carrier aggregation system is described below.

A carrier aggregation system means that a plurality of componentcarriers (CC) is aggregated. The meaning of an existing cell has beenchanged by such a carrier aggregation. In accordance with the carrieraggregation, a cell may mean a combination of downlink CCs and uplinkCCs or a single downlink CC.

Furthermore, in the carrier aggregation, cells may be divided into aprimary cell, a secondary cell, and a serving cell. The primary cellmeans a cell operating in a primary frequency, a cell in which UEperforms an initial connection establishment procedure or connectionreestablishment process on an eNodeB, or a cell designated as a primarycell in a handover process. The secondary cell means a cell operating ina secondary frequency. If an RRC connection is set up, the secondarycell is configured and used to provide additional radio resources.

As described above, in a carrier aggregation system, a plurality ofcomponent carrier (CC), that is, a plurality of serving cells, can besupported unlike in a single carrier system.

Such a carrier aggregation system can support cross-carrier scheduling.Cross-carrier scheduling is a scheduling method for allocating theresource of a PDSCH transmitted through another component carrier and/orallocating the resource of a PUSCH transmitted through a componentcarrier other than a component carrier basically linked to a specificcomponent carrier, through a PDCCH transmitted through the specificcomponent carrier.

<Synchronization Signal>

In LTE/LTE-A systems, synchronization with a cell is obtained through asynchronization signal (SS) in a cell search process.

The synchronization signal is described in detail below with referenceto FIG. 7.

FIG. 7 illustrates a frame structure for the transmission of asynchronization signal in an FDD frame.

A slot number and a subframe number starts with 0. UE may perform timeand frequency synchronization based on a synchronization signal receivedfrom an eNodeB. In 3GPP LTE-A, a synchronization signal is used for cellsearch and may be divided into a primary synchronization signal (PSS)and a secondary synchronization signal (SSS). In 3GPP LTE-A, for asynchronization signal, reference may be made to Paragraph 6.11 of 3GPPTS V10.2.0 (2011-06).

A PSS is used to obtain OFDM symbol synchronization or slotsynchronization and associated with a physical-layer cell identity(PCI). Furthermore, an SSS is used to obtain frame synchronization.Furthermore, an SSS is used to detect a CP length and to obtain aphysical layer cell group ID.

A synchronization signal may be transmitted in a subframe No. 0 and asubframe No. 5 several time by taking into consideration 4.6 ms, thatis, the length of a GSM (global system for mobile communication) framein order to facilitate inter-RAT (radio access technology) measurement.The boundary of the frame may be detected through an SSS. Morespecifically, in an FDD system, a PSS is transmitted in the last OFDMsymbol of a slot No. 1 or a slot No. 10, and an SSS is transmitted in anOFDM symbol right before a PSS.

A synchronization signal may send any one of a total of 504 physicalcell IDs through a combination of three PSSs and 168 SSSs. A PBCH(physical broadcast channel) is transmitted in the first 4 OFDM symbolsof the first slot. A synchronization signal and PBCH are transmittedwithin center 6 Rbs within a system bandwidth so that UE can detect ordemodulate the synchronization signal regardless of a transmissionbandwidth. A physical channel in which a PSS is transmitted is called aP-SCH, and a physical channel in which an SSS is transmitted is calledan S-SCH.

FIG. 8 illustrates an example of a frame structure for sending asynchronization signal in a TDD frame.

In a TDD frame, a PSS is transmitted in the third OFDM symbols of athird slot and thirteenth slot. An SSS is transmitted prior to threeOFDM symbols in OFDM symbols in which a PSS is transmitted. A PBCH istransmitted in the first 4 OFDM symbols of a second slot in the firstsubframe.

<Reference Signal>

A RS is described below.

In general, transmission information, for example, data is easilydistorted and changed while it is transmitted through a radio channel.Accordingly, a reference signal is required in order to demodulate sucha transmission information without an error. The reference signal is asignal known to both a transmitter and a receiver and is transmittedalong with transmission information. Since transmission informationtransmitted by a transmitter experiences a corresponding channel foreach transmission antenna or layer, a reference signal may be allocatedto each transmission antenna or layer. A reference signal for eachtransmission antenna or layer may be identified using resources, such asa frequency and code. A reference signal may be used for two purposes,that is, the demodulation and channel estimation of transmissioninformation.

A downlink reference signal may be divided into a cell-specificreference signal (CRS), an MBSFN (multimedia broadcast and multicastsingle frequency network) reference signal, a UE-specific referencesignal (UE-specific RS, URS), a positioning reference signal(positioning RS, PRS), and a CSI reference signal (CSI-RS). The CRS is areference signal transmitted to all UEs within a cell and also called acommon reference signal. The CRS may be used for the channel measurementof CQI feedback and the channel estimation of PDSCH. The MBSFN referencesignal may be transmitted in a subframe allocated for MBSFNtransmission. The URS is a reference signal received by a specific UE orspecific UE group within a cell and may be called a demodulationreference signal (DM-RS). The DM-RS is chiefly used for a specific UE orspecific UE group to perform data demodulation. The PRS may be used toestimate the location of UE. The CSI-RS is used for the channelestimation of the PDSCH of LTE-A UE. The CSI-RSs are deployed relativelysparsely in a frequency domain or time domain and may be punctured inthe data region of a common subframe or MBSFN subframe.

FIG. 9 illustrates an example of a pattern in which CRSs are mapped toRBs if an eNodeB uses a single antenna port.

Referring to FIG. 9, R0 illustrates an RE to which a CRS transmitted bythe antenna port number 0 of an eNodeB is mapped.

The CRS is transmitted in all downlink subframes within a cell thatsupports PDSCH transmission. The CRS may be transmitted on antenna ports0 to 3. The CRS may be defined only with respect to Δf=15 kHz. Apseudo-random sequence R_(l,ns)(m) generated from a seed value based ona cell ID (identity) is subject to resource mapping as a complex-valuedmodulation symbol a^((p)) _(k,l). In this case, n_(s) is a slot numberwithin a single radio frame, p is an antenna port, and l is an OFDMsymbol number within the slot. K is a subcarrier index. l,k isrepresented as in the following equation.

$\begin{matrix}{{k = {{6m} + {\left( {v + v_{shift}} \right){mod}\mspace{11mu} 6}}}{l = \left\{ {{\begin{matrix}{0,{N_{symb}^{DL} - 3}} & {{{if}\mspace{14mu} p} \in \left\{ {0,1} \right\}} \\1 & {{{if}\mspace{14mu} p} \in \left\{ {2,3} \right\}}\end{matrix}v_{shift}} = {{N_{ID}^{cell}{mod}\mspace{11mu} 6v} = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu} {and}\mspace{14mu} l} = 0}} \\3 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu} {and}\mspace{14mu} l} \neq 0}} \\3 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu} {and}\mspace{14mu} l} = 0}} \\0 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu} {and}\mspace{14mu} l} \neq 0}} \\{3\left( {n_{s}\mspace{11mu} {mod}\mspace{11mu} 2} \right)} & {{{if}\mspace{14mu} p} = 2} \\{3 + {3\left( {n_{s}\mspace{11mu} {mod}\mspace{11mu} 2} \right)}} & {{{if}\mspace{14mu} p} = 3}\end{matrix} \right.}} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, p denotes an antenna port, and n_(s) denotes a slotnumber 0 or 1.

k has 6 shifted indices according to a cell ID (N^(Cell) _(ID)).Accordingly, cells having cell IDs 0, 6, 12, . . . , that is, a multipleof 6, send CRSs in the same subcarrier location k.

In Equation 1, l is determined by the antenna port p, and may have apossible value of 0, 4, 7, or 11. Accordingly, the CRS is transmitted onan 0, 4, 7, 11 symbol.

A resource element (RE) allocated to the CRS of a single antenna portmay not be used to send another antenna port and needs to be configuredto be zero. Furthermore, in an MBSFN (multicast-broadcast singlefrequency network) subframe, the CRS is transmitted only in a non-MBSFNregion.

FIG. 10 illustrates measurement and measurement report procedures.

In a mobile communication system, a mobility support for UE 100 isessential. Accordingly, the UE 100 continues to measure quality of aserving cell that now provides service to the UE 100 and quality ofneighbor cells. The UE 100 reports a measurement result to a network ata proper time, and the network provides optimal mobility to the UEthrough handover. Measurement for such a purpose is called radioresource management (RRM).

The UE 100 may monitor downlink quality of a primary cell (PCell) basedon a CRS. This is called RLM (Radio Link Monitoring). For such RLM, theUE 100 estimates downlink quality and compares the estimated downlinkquality with thresholds, for example, Qout and Qin. The threshold Qoutis defined as a level in which downlink reception cannot be stablyperformed, and corresponds to an 10% error of PDCCH transmission bytaking into consideration a PCFICH error. The threshold Qin is definedas a level in which downlink may be very significantly reliable comparedto the threshold Qout, and corresponds to a 2% error of PDCCHtransmission by taking into consideration a PCFICH error.

As may be seen with reference to FIG. 10, when a serving cell 200 a anda neighbor cell 200 b send respective CRSs (Cell-specific ReferenceSignals) to the UE 100, the UE 100 performs measurement through the CRSsand sends an RRC measurement report message, including a measurementresult, to the serving cell 200 a.

In this case, the UE 100 may perform measurement using the followingthree methods.

1) RSRP (reference signal received power): This indicates the meanreception power of all REs that carry a CRS transmitted in the entireband. In this case, the mean reception power of all REs that carry a CSIRS instead of the CRS may be measured.

2) An RSSI (received signal strength indicator): this indicatesreception power measured in the entire band. The RSSI includes all of asignal, interference, and thermal noise.

3) RSRQ (reference symbol received quality): this indicates a CQI, andmay be determined to be an RSRP/RSSI according to a measurementbandwidth or subband. That is, the RSRQ means an SINR (signal-to-noiseinterference ratio). The RSRP does not provide sufficient mobilityinformation, and thus RSRQ may be used in a handover or cell reselectionprocess instead of RSRP.

RSRQ may be an RSSI/RSSP.

For the measurement, the UE 100 receives a measurement configuration(hereinafter also called “measconfing”) information element (IE) fromthe serving cell 100 a. A message including the measurementconfiguration IE is called a measurement configuration message. In thiscase, the measurement configuration IE may be received through an RRCconnection reconfiguration message. If a measurement result satisfies areport condition within the measconfing information, the UE reports themeasurement result to an eNodeB. A message including the measurementresult is called a measurement report message.

The measurement configuration IE may include measurement objectinformation. The measurement object information is information about anobject on which UE may perform measurement. The measurement objectincludes at least one of an intra-frequency measurement target that isthe subject of intra-cell measurement, an inter-frequency measurementtarget that is the subject of inter-cell measurement, and an inter-RATmeasurement target that is the subject of inter-RAT measurement. Forexample, the intra-frequency measurement target may indicate a neighborcell having the same frequency band as a serving cell. Theinter-frequency measurement target may indicate a neighbor cell having afrequency band different from that of a serving cell. The inter-RATmeasurement target may indicate a neighbor cell having an RAT differentfrom the RAT of a serving cell.

Specifically, the measurement configuration IE includes an IE, such asthat of Table 5.

TABLE 5 MeasConfig ::- -- Measurement objects  measObjectToRemoveList measObjectToAddModList

The Measurement objects IE includes measObjectToRemoveList indicative ofa list of measObject to be removed and measObjectToAddModList indicativeof a list that may be newly added or modified.

measObject includes MeasObjectCDMA2000, MeasObjectEUTRA, andMeasObjectGERAN depending on a communication technology.

An MeasObjectEUTRA IE includes information applied for anintra-frequency or inter-frequency for E-UTRA cell measurement. TheMeasObjectEUTRA IE may be represented as in Table 6.

TABLE 6 1) MeasObjectEUTRA - neighCellConfig -measSubframePatternConfigNeigh-r10 2) MeasSubframePatternConfigNeigh-r10measSubframePatternNeigh-r10 measSubframeCellList-r10

The MeasObjectEUTRA IE may be represented in more detail as follows.

TABLE 7 Description of MeasObjectEUTRA field carrierFreq Thisconfiguration identifies a valid E-UTRA carrier frequency.neighCellConfig indicates information about the configuration of aneighbor cell. measCycleSCell parameter. T_(measure) _(—) _(scc)According to this parameter, a secondary cell (SCell) operates in afrequency indicated by measObject, and this parameter is used in adeactivated state. measSubframeCellList This is a list of cells to whichmeasSubframePatternNeigh is applied. If this is not included, UE appliesa time domain measurement resource restriction pattern to all neighborcells. measSubframePatternNeigh This is a time domain measurementresource restriction pattern applied to measure RSRP and RSRQ onneighbor cells on a carrier frequency indicated by the carrierFreq.

As described above, the MeasObjectEUTRA IE includes information aboutthe configuration (i.e., NeighCellConfig) of a neighbor cell, a timedomain measurement resource restriction pattern (i.e., the measurementsubframe pattern or measSubframePatternNeigh of the neighbor cell)applied to perform measurement on the neighbor cell, and a list of cellsto which the pattern is applied (i.e., measSubframeCellList).

The UE 100 also receives a radio resource configuration IE, asillustrated in FIG. 10.

The radio resource configuration IE is used to configure/modify/releasea radio bearer or to modify a MAC configuration. The radio resourceconfiguration IE includes subframe pattern information. The subframepattern information is information about a measurement resourcerestriction pattern on a time domain in order to measure the RSRP, RSRQof a serving cell (e.g., primary cell).

The radio resource configuration IE includes fields, such as those ofthe following table

TABLE 8 RadioResourceConfigDedicated measSubframePatternPCell-r10

The RadioResourceConfigDedicated field includes the following factors

TABLE 9 Description of RadioResourceConfigDedicated fieldlogicalChannelConfig This is used to indicate whether a logical channelconfiguration is explicitly signaled for SRBs or configured as a defaultlogical channel configuration for SRB1. logicalChannelIdentity A logicalchannel identifier for identifying both uplink (UL) and downlink (DL)mac-MainConfig An option used to indicate whether mac-MainConfig isexplicitly signaled or configured as a default MAC main configuration.measSubframePatternPCell A time domain measurement resource restrictionpattern used to perform measurement (RSRP, RSRQ) on a primary cell(PCell) (i.e., a primary cell) (or a serving cell).

As described above, the RadioResourceConfigDedicated field includesmeasSubframePatternPCell or measSubframePattern-Sere indicative of atime domain measurement resource restriction pattern (i.e., themeasurement subframe pattern of a serving cell) used to performmeasurement (RSRP, RSRQ) on a primary cell (PCell) (or a serving cell).

FIG. 11 illustrates an example of RBs to which CSI-RSs are mapped inreference signals.

A CSI-RS is used for channel measurement for the channel estimation andchannel information of the PDSCH of LTE-A UE. CSI-RSs are deployedrelatively sparsely in a frequency domain or time domain and may bepunctured in the data region of a subframe or MBSFN subframe. If aCSI-RS is required to estimate a CSI, a CQI, PMI, and RI may be reportedby UE.

A CSI-RS is transmitted through a 1, 2, 4, or 8 antenna port. Antennaports used in this case are p=15, p=15, 16, p=15, . . . , 18, and p=15,. . . , 22. That is, a CSI-RS may be transmitted through 1, 2, 4, 8antenna ports. A CSI-RS may be defined with respect to only a subcarrierduration Δf=15 kHz. For a CSI-RS, reference may be made to Paragraph6.10.5 of 3GPP (3rd Generation Partnership Project) TS 36.211 V10.1.0(2011-03) “Technical Specification Group Radio Access Network; EvolvedUniversal Terrestrial Radio Access (E-UTRA); Physical channels andmodulation (Release 8).”

In the transmission for a CSI-RS, a maximum of 32 differentconfigurations may be proposed in order to reduce ICI (inter-cellinterference) in a multi-cell environment including a heterogeneousnetwork (HetNet) environment. A CSI-RS configuration is differentdepending on the number of antenna ports within a cell and a CP. Aneighbor cell may have a different configuration to the greatest extent.Furthermore, a CSI-RS configuration may be divided into a case where itis applied to both an FDD frame and TDD frame and a case where it isapplied to only a TDD frame depending on a frame structure. In a singlecell, a plurality of CSI-RS configurations may be used. A zero or oneCSI-RS configuration may be used for UE that assumes a non-zero powerCSI-RS, and zero or some CSI-RS configurations may be used for UE thatassumes a zero power CSI-RS.

A CSI-RS configuration may be indicated by a high layer. For example, aCSI-RS-Config IE (information element) transmitted through a high layermay indicate a CSI-RS configuration. The following table illustrates anexample of a CSI-RS-Config IE.

TABLE 10 CSI-RS-Config-r10 ::= SEQUENCE {  csi-RS-r10 CHOICE {   releaseNULL,   setup SEQUENCE {    antennaPortsCount-r10   ENUMERATED {an1,an2, an4, an8},    resourceConfig-r10   INTEGER (0..31),   subframeConfig-r10   INTEGER (0..154),    p-C-r10     INTEGER(−8..15)   }  }  OPTIONAL, -- Need ON  zeroTxPowerCSI-RS-r10 CHOICE {  release NULL,   setup SEQUENCE {    zeroTxPowerResourceConfigList-r10 BIT STRING (SIZE (16)),    zeroTxPowerSubframeConfig-r10    INTEGER(0..154)   }  }  OPTIONAL -- Need ON } -- ASN1STOP

Referring to Table 10, an “antennaPortsCount” field indicates the numberof antenna ports used for the transmission of a CSI-RS. A“resourceConfig” field indicates a CSI-RS configuration. A“SubframeConfig” field and a “zeroTxPowerSubframeConfig” field indicatea subframe configuration in which a CSI-RS is transmitted.

A “zeroTxPowerResourceConfigList” field indicates the configuration of azero power CSI-RS. In a 16-bit bitmap that configures thezeroTxPowerResourceConfigList” fiel, a CSI-RS configurationcorresponding to bits configured to be 1 may be configured as a zeropower CSI-RS.

The sequence r_(l,ns)(m) of a CSI-RS may be generated as in thefollowing equation.

$\begin{matrix}{{{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}},\mspace{20mu} {m = 0},\ldots \mspace{14mu},{N_{RB}^{\max,{DL}} - {1\mspace{14mu} {where}}},{c_{init} = {{2^{10} \cdot \left( {{7 \cdot \left( {n_{s} + 1} \right)} + l + 1} \right) \cdot \left( {{2 \cdot N_{ID}^{cell}} + 1} \right)} + {2 \cdot N_{ID}^{cell}} + N_{CP}}}}\mspace{20mu} {N_{CP} = \left\{ \begin{matrix}1 & {{for}\mspace{14mu} {normalCP}} \\0 & {{for}\mspace{14mu} {extendedCP}}\end{matrix} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, n_(s) is a slot number within a radio frame, and l is anOFDM symbol number within the slot. c(i) is a pseudo random sequence andstarted from each OFDM symbol as c_(init) indicated in Equation. N_(ID)^(cell) means a physical cell ID.

In subframes configured to send a CSI-RS, a reference signal sequencer_(l,ns)(m) is mapped to a complex value modulation symbols a_(k,l)^((p)) used as a reference symbol for an antenna port p.

The relation between r_(l,ns)(m) and a_(k,l) ^((p)) may be representedas in the following equation.

a _(k,l) ^((p)) =w _(l″) ·r(m)  [Equation 3]

In this case,

$k = {k^{\prime} + {12m} + \left\{ {{\begin{matrix}{- 0} & {{{{for}\mspace{14mu} p} \in \left\{ {15,16} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 6} & {{{{for}\mspace{14mu} p} \in \left\{ {17,18} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 1} & {{{{for}\mspace{14mu} p} \in \left\{ {19,20} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 7} & {{{{for}\mspace{14mu} p} \in \left\{ {21,22} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 0} & {{{{for}\mspace{14mu} p} \in \left\{ {15,16} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 3} & {{{{for}\mspace{14mu} p} \in \left\{ {17,18} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 6} & {{{{for}\mspace{14mu} p} \in \left\{ {19,20} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 9} & {{{{for}\mspace{14mu} p} \in \left\{ {21,22} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}}\end{matrix}l} = {l^{\prime} + \left\{ {{\begin{matrix}l^{''} & \begin{matrix}{{CSI}\mspace{14mu} {reference}\mspace{14mu} {signal}} \\{{{configurations}\mspace{14mu} 0\text{-}19},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}}\end{matrix} \\{2l^{''}} & \begin{matrix}{{CSI}\mspace{14mu} {reference}\mspace{14mu} {signal}} \\{{{configurations}\mspace{14mu} 20\text{-}31},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}}\end{matrix} \\l^{''} & \begin{matrix}{{CSI}\mspace{14mu} {reference}\mspace{14mu} {signal}} \\{{{configurations}\mspace{14mu} 0\text{-}27},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}}\end{matrix}\end{matrix}w_{l^{''}}} = \left\{ {{{\begin{matrix}1 & {p \in \left\{ {15,17,19,21} \right\}} \\\left( {- 1} \right)^{l^{''}} & {p \in \left\{ {16,18,20,22} \right\}}\end{matrix}l^{''}} = 0},{{1m} = 0},1,\ldots \mspace{14mu},{{N_{RB}^{DL} - {1m^{\prime}}} = {m + \left\lfloor \frac{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}{2} \right\rfloor}}} \right.} \right.}} \right.}$

In Equation 2, (k″, l″) and n_(s) are given in Table 5 and Table 6 to bedescribed later. A CSI-RS may be transmitted in a downlink slot in which(n_(s) mod 2) satisfies the conditions of Table 5 and Table 6 (In thiscase, mod means modular operation. That is, (n_(s) mod 2) means theremainder obtained by dividing n_(s) by 2).

The following table illustrates CSI-RS configurations in a normal CP.

TABLE 11 Number of configured CSI-RSs 1 or 2 4 8 CSI-RS n_(s) n_(s)n_(s) configuration mod mod mod index (k′, 1′) 2 (k′, 1′) 2 (k′, 1′) 2TDD 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 and 1 (11, 2) 1 (11, 2) 1 (11, 2) 1 FDD2 (9, 2) 1 (9, 2) 1 (9, 2) 1 frame 3 (7, 2) 1 (7, 2) 1 (7, 2) 1 4 (9, 5)1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8, 5) 0 6 (10, 2) 1 (10, 2) 1 7 (8, 2) 1(8, 2) 1 8 (6, 2) 1 (6, 2) 1 9 (8, 5) 1 (8, 5) 1 10 (3, 5) 0 11 (2, 5) 012 (5, 2) 1 13 (4, 2) 1 14 (3, 2) I 15 (2, 2) 1 16 (1, 2) 1 17 (0, 2) 118 (3, 5) 1 19 (2, 5) 1 TDD 20 (11, 1) 1 (11, 1) 1 (11, 1) 1 frame 21(9, 1) 1 (9, 1) 1 (9, 1) 1 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 23 (10, 1) 1(10, 1) 1 24 (8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27(4, 1) 1 28 (3, 1) 1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1

The following table illustrates CSI-RS configuration in an extended CP.

TABLE 12 Number of configured CSI-RSs 1 or 2 4 8 CSI-RS n_(s) n_(s)n_(s) configuration mod mod mod index (k′, 1′) 2 (k′, 1′) 2 (k′, 1′) 2TDD 0 (11, 4) 0 (11, 4) 0 (11, 4) 0 and 1 (9, 4) 0 (9, 4) 0 (9, 4) 0 FDD2 (10, 4) 1 (10, 4) 1 (10, 4) 1 frame 3 (9, 4) 1 (9, 4) 1 (9, 4) 1 4 (5,4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4) 0 6 (4, 4) 1 (4, 4) 1 7 (3, 4) 1 (3, 4)1 8 (8, 4) 0 9 (6, 4) 0 10 (2, 4) 0 11 (0, 4) 0 12 (7, 4) 1 13 (6, 4) 114 (1, 4) 1 15 (0, 4) 1 TDD 16 (11, 1) 1 (11, 1) 1 (11, 1) 1 frame 17(10, 1) 1 (10, 1) 1 (10, 1) 1 18 (9, 1) 1 (9, 1) 1 (9, 1) 1 19 (5, 1) 1(5, 1) 1 20 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 1 22 (8, 1) 1 23 (7, 1)1 24 (6, 1) 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

In the above two tables, UE may send a CSI-RS only in a downlink slotthat satisfies the condition of n_(s) mod 2. Furthermore, UE does notsend a CSI-RS in a subframe in which the transmission of a specialsubframe, CSI-RS of a TDD frame collides against a synchronizationsignal, a PBCH (physical broadcast channel), and a system informationblock type 1 (SystemInformationBlockType1) or a subframe in which apaging message is transmitted. Furthermore, in a se S, that is, S={15},S={15, 16}, S={17, 18}, S={19, 20} or S={21, 22}, a resource element inwhich the CSI-RS of a single antenna port is transmitted is not used forthe transmission of the CSI-RS of a PDSCH or another antenna port.

The following table illustrates an example of a subframe configurationin which a CSI-RS is transmitted.

TABLE 13 CSI-RS- CSI-RS SubframeConfig CSI-RS cycle subframe offsetI_(CSI-RS) T_(CSI-RS) (subframe) Δ_(CSI-RS) (subframes) 0-4 5 I_(CSI-RS) 5-14 10 I_(CSI-RS) − 5  15-34 20 I_(CSI-RS) − 15 35-74 40 I_(CSI-RS) −35  75-154 80 I_(CSI-RS) − 75

Referring to Table 13, the cycle T_(CSI-RS) and offset Δ_(CSI-RS) of asubframe in which a CSI-RS is transmitted may be determined depending ona CSI-RS subframe configuration I_(CSI-RS). In Table 13, the CSI-RSsubframe configuration may be any one of the “SubframeConfig” field and“ZeroTxPowerSubframeConfig” field of the CSI-RS-Config IE in the abovetable. The CSI-RS subframe configuration may be separately configuredwith respect to a non-zero power CSI-RS and a zero power CSI-RS.

FIG. 11 illustrates resource elements used for CSI-RSs when a CSI-RSconfiguration index is 0 in a normal CP structure. Rp illustrates aresource element used for CSI-RS transmission on an antenna port p.Referring to FIG. 11, a CSI-RS for antenna ports 15 and 16 istransmitted through resource elements corresponding to the thirdsubcarrier (i.e., subcarrier index 2) of the sixth and the seventh OFDMsymbols (i.e., OFDM symbol indices 5, 6) of a first slot. A CSI-RS forantenna ports 17 and 18 is transmitted through resource elementscorresponding to the ninth subcarrier (i.e., subcarrier index 8) of thesixth and the seventh OFDM symbols (OFDM symbols indices 5, 6) of thefirst slot. A CSI-RS for antenna ports 19 and 20 is transmitted throughthe same resource elements as those in which the CSI-RS for the antennaports 15 and 16 is transmitted. A CSI-RS for the antenna ports 21 and 22is transmitted through the same resource elements as those in which theCSI-RS for the antenna ports 17 and 18 are transmitted.

IF a CSI-RS through eight antenna ports is transmitted to UE, the UE mayreceive an RB to which R15 to R22 has been mapped. That is, the UE mayreceive a CSI-RS having a specific pattern.

Hereinafter, a small cell is described below.

<Introduction of Small Cell>

In a next-generation mobile communication system, it is expected that asmall cell having a small coverage radius will be added to the coverageof an existing cell and a small cell may process more traffic. Theexisting cell is called a macro cell because it has greater coveragethan the small cell. This is described below with reference to FIG. 7.

FIG. 12 is a diagram illustrating a heterogeneous network environment inwhich a macro cell and small cells having a possibility that they maybecome a next-generation wireless communication system are mixed.

Referring to FIG. 12, a macro cell according to an existing eNodeB 200has a heterogeneous network environment in which overlaps with smallcells according to one or more small eNodeBs 300 a, 300 b, 300 c, and300 d. The existing eNodeB is also called a macro eNodeB (MeNB) becauseit provides coverage greater than the small eNodeB. In thisspecification, a macro cell and a macro eNodeB are interchangeably used.UE connected to the macro cell 200 may be called macro UE. The macro UEreceives a downlink signal from a macro eNodeB and sends an uplinksignal to a macro eNodeB.

In such a heterogeneous network, the coverage gap of a macro cell may befilled by configuring the macro cell as a primary cell (PCell) andconfiguring the small cell as a secondary cell (SCell). Furthermore,overall performance can be boosted by configuring a small cell as aprimary cell (PCell) and configuring a macro cell as a secondary cell(SCell).

However, inter-cell interference may be further added due to theintroduction of such small cells.

The most fundamental method for solving such an interference problem isto differently use frequencies between cells. However, the frequenciesare rare and expensive resources, and thus a method through frequencypartition is not welcomed by providers.

Accordingly, in 3GPP, such an inter-cell interference problem has beenintended to be solved through time partition.

Accordingly, in recent 3GPP, active research is being carried out oneICIC (enhanced inter-cell interference coordination) as one ofinterference cooperation methods.

<Introduction of eICIC>

A time partition method introduced into LTE Release-10 is called anenhanced ICIC (Enhanced inter-cell interference Coordination) meaningthat it has been advanced from the existing frequency partition method.In this method, a cell that generates interference is called anaggressor cell or primary cell, and a cell subjected to interference iscalled a victim cell or secondary cell. In a specific subframe, anaggressor cell or primary cell stops data transmission so that UE canmaintain access to a victim cell or secondary cell in the correspondingsubframe. That is, in this method, if heterogeneous cells coexist, acell on one side rarely sends an interference signal because ittemporarily stops the transmission of a signal to UE subjected to verygreat interference in any region.

A specific subframe in which data transmission is stopped is called anABS (Almost Blank Subframe). In subframe corresponding to the ABS, anydata is not transmitted other than essential control information. Theessential control information may be a CRS, for example. Accordingly,data is not transmitted on a subframe to which an ABS has been applied,but only a CRS signal is transmitted on a symbol No. 0, 4, 7, 11.

FIG. 13 is an exemplary diagram of eICIC (enhanced Inter-CellInterference Coordination) for solving interference between eNodeBs.

Referring to FIG. 13, the eNodeB 200 of a macro cell performs datatransmission in the data region of an illustrated subframe.

In this case, in order to solve interference, the eNodeB 300 of a smallcell uses an eICIC. That is, if the eICIC is applied, a correspondingsubframe is managed depending on an ABS, and any data may not betransmitted in the data region.

However, in the subframe managed depending on the ABS, only a CRS may betransmitted on a symbol No. 0, 4, 7, 11.

If small cells are deployed as described above, an inter-cellinterference problem may be worsened. In order to solve this problem, asillustrated in FIG. 13, the coverage size of a small cell may be reduceddepending on the situation. Alternatively, the small cell may be off andthen on depending on the situation.

FIG. 14 is an exemplary diagram illustrating the situation in whichsmall cells have been densely deployed.

FIG. 14 illustrates the state in which small cells have been denselydeployed within the coverage of a macro cell. In this state, it may bedifficult for UE 100 to detect the small cells within a short time. Inparticular, as described above, cell detection is performed through thereception of a PSS/SSS. However, if many small cells send PSS/SSSs atthe same timing, that is, on subframes Nos. 0 and 5, it may be difficultfor the UE 100 to receive all the PSS/SSSs at once. Furthermore, if thesmall cells send the PSS/SSSs on the subframes Nos. 0 and 5 at the sametime, mutual interference may be generated. As a result, it may bedifficult for the UE 100 to correct receive the PSS/SSSs.

<Disclosure of this Specifications>

Accordingly, a disclosure of this specification is to propose a schemefor solving such a problem.

FIG. 15 illustrates an example in which small cells send discoverysignals according to the disclosure of this specification.

In order to solve the problem, as may be seen with reference to FIG. 15,a disclosure of this specification proposes a method in which in orderfor UE to efficiently detect small cells, a small cell sends a newdiscovery signal (DS) in addition to an existing PSS/SSS. The discoverysignal may also be called a discovery reference signal (DRS).Accordingly, the UE needs to perform a cell search process or celldetection process using the discovery signal in addition to the existingPSS/SSS.

In this case, the discovery signal may mean a signal that has a longcycle and that is cyclically transmitted.

Such a discovery signal may be transmitted by an RRH (remote radio head)or a transmission point in addition to a small cell.

The discovery signal may have the following characteristics.

-   -   It allows more cells to be detected compared to existing PSS/SSS        and a CRS.    -   It allows more cells to be detected for a short time, for        example, during a single subframe.    -   It allows measurement to be performed for a short time, for        example, during a single subframe.    -   It supports the measurement of a small cell that performs on/off        operations. In other words, although a small cell is in the OFF        state, the small cell sends a discovery signal so that UE may        perform measurement based on the discovery signal.

A discovery signal may be implemented as the following signals.

(a) PSS/SSS/CSI-RS/CRS or PSS/SSS/configurable CRS

(b) PSS/SSS/CRS

(c) PSS/SSS/CSI-RS

(d) PSS/SSS/CSI-RS/CRS or PSS/SSS/configurable CSI-RS

Such a discovery signal may be used for coarse time/frequency trackingand measurement.

A discovery signal needs to satisfy the following requirements.

-   -   It needs to support coarse time synchronization assuming that an        initial timing error is very high (e.g., +−2.5 ms).    -   It needs to support coarse frequency synchronization assuming        that an initial frequency error is very high (e.g., 20 kHz).    -   It needs to support the detection of at least three cells.

The cycle of a discovery signal is determined by taking intoconsideration the following restrictions.

-   -   Several measurement gap durations (or measurement gap periods):        40 msec, 80 msec, 160 msec, or 320 msec.    -   A DRX cycle and alignment: 10, 20, 32, 40, 64, 80, 128, 160,        256, 320, 512, 640, 1024, 1280, 2048, 2560.    -   If a PSS/SSS is transmitted as part of a discovery signal, the        cycle of the discovery signal becomes a multiple of 5 msec. A        common PSS/SSS transmitted in the ON state needs to be        substituted with the PSS/SSS of a discovery signal. However,        such a restriction may not be applied if a small cell does not        send a discovery signal in the ON state. Alternatively, in order        to minimize the influence of existing UE other than UE improved        according to the disclosure of this specification, a PSS/SSS for        a discovery signal in addition to the existing PSS/SSS may be        separately transmitted. As described above, the PSS/SSS        separately transmitted for a discovery signal in addition to an        existing PSS/SSS may be called a DS-PSS (or DRS-PSS)/DS-SSS (or        DRS-SSS). In this case, a cell ID that is a base for the DS-PSS        (or DRS-PSS)/DS-SSS (or DRS-SSS) may be different from a cell ID        that is a base for the PSS/SSS.

If one or more of a CRS and a CSI-RS are separately transmitted for adiscovery signal in addition to an existing CRS, such a CRS and CSI-RSmay be respectively called a DS-CRS (or DRS-CRS) and DS-CSI-RS (orDRS-CSI-RS). Furthermore, if a PRS is separately transmitted for adiscovery signal in addition to an existing PRS, such as a PRS may becalled a DS-PRS (or DRS-PRS).

Furthermore, in this specification, a DRS-PSS, a DRS-SSS, a DRS-CRS, aDRS-CSI-RS, and a DRS-PRS mean respective PSS, SSS, CRS, CSI-RS, and PRSincluded in respective discovery signals.

If a DRS transmitted by a specific cell in a long cycle has one of theaforementioned (a)-(d) types, the sequence and resources of a DRS-PSS,DRS-SSS, DRS-CRS, and DRS-CSI-RS may be transmitted in forms mostsimilar to the existing PSS, SSS, CRS, and CSI-RS, but may be differentfrom the conventional PSS, SSS, CRS, and CSI-RS in such a manner thatthey are transmitted on other scrambling initial parameters and/orresource locations (e.g., other frequency/time resources). Morespecifically, a DRS-CSI-RS may use the resources pattern of an existingCSI-RS, but may have a different transmission subframe and cycle orscrambling ID. That is, the scrambling ID of a DRS-CSI-RS and CSI-RStransmitted by a specific cell, the number of antenna ports, and atransmission cycle/offset may be different.

FIG. 16 illustrates an example in which a plurality of transmissionpoints (TPs) (or small cells) within a cluster uses the same physicalcell identifier (PCID).

As may be seen with reference to FIG. 16, a plurality of transmissionpoints (or a small cell) is grouped for each cluster, and transmissionpoints (or small cells) within each cluster may use the same physicalcell identifier (PCID) as its own macro eNodeB. Such an environment maybe called a sharing cell-ID scenario. In this case, the PCID may mean acell-unique ID used for PSS/SSS and CRS transmission as in a current LTEtechnology or may be a separate cluster ID in common used in a specificcluster.

In such an environment, in order to obtain an additional cell-splittinggain between a plurality of transmission points within a cluster, uniqueID information may be assigned to each transmission point. As describedabove, unique ID information for each transmission point may be called atransmission point ID. As a representative embodiment, each transmissionpoint ID may be used as the sequence scrambling-initial parameter (e.g.,scramblingldentity) of any one of a CSI-RS or discovery signaltransmitted by a corresponding transmission point and may be used in thetransmission of a unique reference signal (RS) for each othertransmission point.

In this specification, the state in which each transmission point sendsa unique discovery reference signal (DRS) for each unique transmissionpoint is taken into consideration. A DRS may be configured as severalRSs. Each transmission point is not assumed to send several RSs. Forexample, if it is assumed that a DRS is configured as aDRS-PSS/DRS-SSS/DRS-CSI-RS/DRS-CRS, the DRS-PSS/DRS-SSS/DRS-CRS may betransmitted in each transmission point or may be transmitted inrepresentative transmission points.

One of roles performed by UEs through a discovery signal is RSRP/RSRQmeasurement as described above. In an existing system, UE performs RSRPmeasurement and RSRQ measurement through a CRS. The same is true of themeasurement of a small cell. In this case, UE may perform measurement ona small cell that sends a discovery signal through the discovery signal.However, since a CRS and a DRS may have different sequences, RElocations, and RE densities, the value of RSRP, RSRQ measured through aCRS may be different from the value of RSRP, RSRQ measured through adiscovery signal with respect to the same small cell. Hereinafter, forconvenience of description, the value of RSRP, RSRQ measured using a CRSin a prior art is respectively called C-RSRP, C-RSRQ. Unlike in theprior art, RSRP, RSRQ measured through a discovery signal isrespectively called D-RSRP, D-RSRQ.

UE may receive a DRS measurement timing configuration (DMTC), that is,timing information for DRS-based measurement, from an eNodeB. The DMTCmay be included in “measobject” within the measurement configuration(measconfig) and received. Such a DMTC may include a cycle and an offsetvalue and may additionally include the value of duration.

If an ABS is managed in order to reduce inter-cell interference, UE isunaware that which subframe has been configured as an ABS. For example,if an aggressor cell has configured an ABS, an interference level isgreatly changed for each subframe. Accordingly, some UEs may not receiveresources allocated thereto on specific subframes. If UE does notdistinguish between a subframe in which an ABS has been configured and asubframe in which an ABS has not been configured, the UE needs to simplytake the mean of interference levels that have been severely changed foreach subframe and report the mean. Accordingly, an inaccuratemeasurement result is reported.

In order to solve such a problem, the aforementioned time domainmeasurement resource restriction pattern, that is, a measurementsubframe pattern, may be used. By sending information about such ameasurement subframe pattern to UE, the UE may perform measurement onlyon a subframe of a specific pattern.

If a neighbor small cell has performed On/Off operations and thus it isdifficult for UE to perform measurement on the neighbor small cell usingan existing CRS, the UE may perform measurement using a discovery signalfrom the neighbor small cell. In this case, however, a problem may occurbecause it is not clear whether UE has to perform measurement on a smallcell, operating in the ON state as the serving cell of the UE, usingwhich one of the CRS and the discovery signal.

If another small cell generates a discovery signal through a combinationof a PSS/SSS/CRS (i.e., DRS-PSS/DRS-SSS/DRS-CRS) or a PSS/SSS/CSI-RS(i.e., DRS-PSS/DRS-SSS/DRS-CSI-RS) and sends the discovery signal on aplurality of subframes (e.g., 6 or 10 subframes), but configures some ofthe subframes as ABSs, a problem may occur because it is not clearwhether UE has to perform measurement only on a limited subframeindicated by a measurement subframe pattern with respect to thediscovery signal.

In accordance with another existing definition, an RSSI is calculatedbased on a measurement result on a specific OFDM symbol including a CRS.However, if such an existing definition is directly applied to adiscovery signal, a problem may occur. The reason for this is describedbelow. First, if the existing definition is also applied to thediscovery signal, the RSSI of the discovery signal is calculated basedon a measurement result only on an OFDM symbol including the discoverysignal. However, it is assumed that a neighbor small cell sends adiscovery signal in OFF state. In this case, since no data istransmitted on a downlink subframe, a measured RSSI is inaccurate.Accordingly, there is a problem in that the calculation of RSRQ becomesinaccurate.

Schemes for solving the aforementioned problems are described below.

I. Reference Signal for RSRP/RSRQ Measurement of Serving Cell

First, an example in which UE has to perform RSRP/RSRQ measurement on aserving cell that belongs to the serving cells of the UE and that maysend a discovery signal using which one of a CRS and a discovery signalis described below. In this case, if specifically described in thefollowing example, the serving cell of the UE may be understood to be aprimary cell (PCell).

First, if the discovery signal measurement timing configuration (DMTC)of each serving cell has not been received, UE may perform CRS-basedRSRP/RSRQ measurement. In this case, the DMTC means that a serving cellconfigures a subframe on which UE may perform measurement is configuredin the UE. The DMTC may include a subframe cycle “ubframeperiod”, asubframe offset, and/or subframe duration.

If UE has received a DMTC for each serving cell, the UE may operateaccording to any one of the following schemes.

In a first scheme, while UE is connected to a serving cell, the servingcell is always in the ON state. Accordingly, the UE may always performCRS-based RSRP/RSRQ measurement on its own serving cell. That is,although the serving cell of the UE sends a discovery signal, the UEperforms RSRP/RSRQ measurement based on a CRS other than a discoverysignal. In other words, when performing an RSRP/RSRQ report on itsserving cell, the UE may report only a C-RSRP, C-RSRQ value. Incontrast, the UE may perform discovery signal-based RSRP/RSRQmeasurement only when it tries to perform measurement on a neighborcell. Such a first scheme is described in more detail below withreference to the following figures.

FIG. 17a is an exemplary diagram of the first solution regarding thatmeasurement will be performed using which one of a CRS and a DS.

As may be seen with reference to FIG. 17a , if UE receives a discoverysignal measurement timing configuration (DMTC), the UE may performmeasurement on a second cell using the DMTC. In contrast, the UE mayperform CRS-based measurement without applying the DMTC to a first cell.

In this case, the first cell means a primary cell (PCell) of the servingcells of the UE. Furthermore, the second cell means a cell other thanthe primary cell and includes, for example, a secondary cell (SCell) ora neighbor cell.

If the first cell is the primary cell (PCell), the UE may receive themeasurement subframe pattern (e.g., measSubframePatternPCell) of theprimary cell (PCell). In this case, the UE may perform CRS-basedmeasurement on the primary cell (PCell) by applying the measurementsubframe pattern. In other words, the UE may perform CRS-basedmeasurement on a subframe indicated by the measurement subframe patternof the primary cell (PCell).

FIG. 17b is a more detailed exemplary diagram of the first solutionregarding that measurement will be performed using which one of a CRSand a DS.

Referring to FIG. 17b , UE receives a measurement subframe pattern. Inthis case, the reception includes both the reception of the measurementsubframe pattern of a primary cell (PCell) and the reception of themeasurement subframe pattern of a neighbor cell.

Accordingly, the UE performs CRS-based measurement by applying themeasurement subframe pattern to a first cell, for example, a primarycell (PCell). That is, the UE performs measurement using a CRS receivedfrom the first cell, for example, the primary cell (PCell) on a subframeindicated by the measurement subframe pattern.

If the UE receives a DMTC, the UE performs measurement on a second cellby applying the DMTC. If the UE does not receive a DMTC, the UE performsCRS-based measurement by applying a measurement subframe pattern.Specifically, if the UE does not receive a DMTC, the UE performsmeasurement using a CRS received from the second cell on a subframeindicated by a measurement subframe pattern.

A second solution and third solution are described below.

In accordance with the second scheme, UE may perform discoverysignal-based RSRP/RSRQ measurement on a serving cell. In this case, theUE does not distinguish between the serving cell and a neighbor cell. Ifthe UE receives the DMTC of a specific cell, the UE may performdiscovery signal-based RSRP/RSRQ measurement. In this case, the UE mayreport only D-RSRP and D-RSRQ values when reporting RSRP/RSRQmeasurement results of its own serving cell.

In accordance with the third scheme, UE may perfon CRS-based RSRP/RSRQmeasurement or may perform discovery signal-based RSRP/RSRQ measurement.That is, the UE may perform CRS- or discovery signal-based measurementor may perform both CRS-based measurement and discovery signal-basedmeasurement based on a specific-RS without being limited to specificmeasurement. In this case, the UE may report a C-RSRP/C-RSRQ and/orD-RSRP/D-RSRQ value to an eNodeB. In this case, the UE may reportRSRP/RSRQ values to the eNodeB and may also report whether correspondingRSRP/RSRQ values is a CRS-based measurement value or a discoverysignal-based measurement value.

In this case, when reporting the RSRP/RSRQ values to the eNodeB, the UEmay transfer information about whether the corresponding RSRP/RSRQ valueis a CRS-based measurement value or a discovery signal-based measurementvalue.

If UE may perform discovery signal-based RSRP/RSRQ measurement, both aDS-CRS and a DS-CSI-RS may have been included in the discovery signal.In this case, the UE may use the DS-CRS or DS-CSI-RS for RSRP/RSRQmeasurement. Alternatively, the UE may use both the DS-CRS andDS-CSI-RS.

Whether UE has to perform RSRP/RSRQ measurement on a specifictransmission point using a DS-CRS or a DS-CSI-RS may be differentdepending on a cell ID operation method (i.e., a sharing cell IDoperation method or a non-sharing cell ID operation method). In thenon-sharing cell ID operation method, a DS-CRS (and DS-CSI-RS) that isdifferent (distinguished) for each transmission point is transmitted. Inthe sharing cell ID operation method, the same DS-CRS (notdistinguished) between transmission points is transmitted and only aDS-CSI-RS is transmitted (so that it is distinguished). In this case, UEmay not determine whether it has to perform RSRP/RSRQ measurement usingwhich RS because it is unable to know that a specific cell ortransmission point operates in the sharing cell ID operation method orthe non-sharing cell ID operation method.

To this end, an eNodeB may notify the UE whether the UE has to performthe RSRP/RSRQ measurement using the DS-CRS (or CRS) or the DS-CSI-RS (orCSI-RS) through high layer signaling. If an eNodeB has configured thatUE has to perform RSRP/RSRQ measurement using a DS-CRS through highlayer signaling, the UE may perform the RSRP/RSRQ measurement using theDS-CRS and report a measurement result to the eNodeB. Alternatively, ifthe eNodeB has configured that the UE has to perform the RSRP/RSRQmeasurement using a DS-CSI-RS through high layer signaling, the UE mayperform the RSRP/RSRQ measurement using the DS-CSI-RS and report ameasurement result to the eNodeB.

II. RSRP/RSRQ Measurement if Measurement Subframe is Configured

As described above, if UE receives a DMTC and also receives ameasurement subframe pattern, there is a problem in that the UE has toperform measurement on which subframe. More specifically, if UE receivesa DMTC, the UE has to perform measurement on a subframe indicated in theDMTC. Furthermore, if UE receives a measurement subframe pattern, the UEneeds to perform measurement on a subframe indicated by the measurementsubframe pattern. However, if a subframe indicated by the DMTC is notexactly identical with a subframe indicated by the measurement subframepattern, it is not clear that the UE has to perform measurement on whichsubframe.

Solutions for such a problem are described below.

First, the following solutions may be based on a precondition in which ameasurement subframe pattern has been configured based on the timing ofa serving cell so that UE may perform restricted measurement although itis unaware of the SFN of a neighbor cell and a subframe index by takinginto consideration the state in which timing is not synchronized betweenthe serving cell and neighbor cell of the UE (i.e., an asynchronouscase). In this case, the serving cell may be the primary cell (PCell) ofthe UE or the PCell of a secondary cell group (SCG), a specificsecondary cell (Scell), or a cell that sends assistance information bytaking into consideration an environment, such as a CA or dualconnectivity.

A first solution is an example in which a discovery signal includes aPSS/SSS/CSI-RS or a PSS/SSS/CRS/CSI-RS. As described above, if adiscovery signal includes a PSS/SSS/CSI-RS or a PSS/SSS/CRS/CSI-RS, UEmay perform RSRP/RSRQ measurement through a corresponding CSI-RS (i.e.,DS-CSI-RS). In this case, in order to reduce interference, theDS-CSI-RSs may be transmitted using different scrambling indices and/orRE locations between neighboring cells or transmission points so thatthey are orthogonal to each other. Accordingly, in this case, the UEdoes not need to perform restricted measurement on a subframe indicatedin a measurement subframe pattern received from an eNodeB. Accordingly,the first solution proposes that if UE performs discovery signal-basedRSRP/RSRQ measurement (if D-RSRP, D-RSRQ are measured), the UE neglectsa measurement subframe pattern although the UE receives the measurementsubframe pattern and performs measurement. In this case, the UE mayapply the measurement subframe pattern, that is, usemeasSubframePatternPCell, measSubframePatternNeigh as a CRS only whenperforming RSRP/RSRQ measurement.

A second solution is an example in which a discovery signal includes aPSS/SSS/CRS or a PSS/SSS/CRS/CSI-RS.

If a discovery signal includes a PSS/SSS/CRS, UE may perform RSRP/RSRQmeasurement through a corresponding CRS (i.e., DS-CRS). Alternatively,if a discovery signal includes a PSS/SSS/CRS/CSI-RS, UE may performRSRP/RSRQ measurement using a DS-CRS and/or a DS-CSI-RS. In this state,if the UE has received a measurement subframe pattern and configured themeasurement subframe pattern, the UE may perform RSRP/RSRQ measurementas follows. In the following contents, an example in which a DS-CRS isincluded in a discovery signal is described, but the present inventionmay also be applied to a case where a DS-CRS is not included in adiscovery signal, but only a DS-CSI-RS is included in the discoverysignal (i.e., the discovery signal includes a PSS/SSS/CSI-RS).

In the first embodiment of the second solution, UE may perform discoverysignal-based measurement on a subframe overlapping with a subframe thatbelongs to subframes indicated by a measurement subframe pattern andthat is indicated by a DMTC. That is, although UE measures RSRP/RSRQthrough a discovery signal according to a DMTC, the UE needs to performmeasurement on a restricted subframe indicated by a measurement subframepattern. In other words, although UE performs RSRP/RSRQ measurementthrough a discovery signal according to a DMTC as well as a CRS, ameasurement subframe pattern, that is, measSubframePatternPCell,measSubframePatternNeigh, may be applied. A detailed flow is describedbelow with reference to FIG. 18.

FIG. 18 illustrates a process of determining a subframe on which UE willperform measurement if both a measurement subframe pattern and a DMTCare used.

As may be seen with reference to FIG. 18, UE receives a measurementsubframe pattern. Furthermore, if the UE also receives a DMTC, the UEselects a subframe on which measurement is to be performed based on boththe measurement subframe pattern and the DMTC and performs measurementon the selected subframe. Specifically, the UE selects a subframeoverlapping with a subframe that belongs to subframes indicated by themeasurement subframe pattern and that is indicated by the DMTC. This isdescribed below with reference to FIGS. 19a and 19 b.

FIGS. 19a and 19b illustrate examples in which a subframe on whichmeasurement is to be performed based on both a measurement subframepattern and a DMTC.

As illustrated in FIG. 19a , in accordance with discovery signalmeasurement configuration (DMTC) timing, a discovery signal may bereceived on a plurality of subframes (e.g., 6 subframes). In this case,a DS-PSS, DS-SSS may be received on some (e.g., one) of the subframes oron all of the subframes, but a DS-CSI-RS may be received on all thesubframes.

Referring to FIG. 19b , UE may perform measurement only on a subframeoverlapping with a subframe that belongs to subframes indicated by aDMTC and that is indicated by a measurement subframe pattern.

In the second embodiment of the second solution, by taking intoconsideration that a discovery signal is transmitted through a smallnumber of subframes, for example, one subframe, although a DS-CRS isincluded in the discovery signal, to perform measurement only on arestricted subframe indicated by a measurement subframe pattern may bemeaningless. Accordingly, if UE performs RSRP/RSRQ measurement using adiscovery signal (i.e., if D-RSRP, D-RSRQ is measured), although the UEhas received a measurement subframe pattern, the UE may neglect themeasurement subframe pattern and perform measurement. In this case, themeasurement subframe pattern, that is, measSubframePatternPCell,measSubframePatternNeigh, may be applied only when the UE performsRSRP/RSRQ measurement using a CRS. More specifically, ifmeasSubframePatternNeigh is configured, the UE may perform CRS-basedmeasurement, assuming that the UE sends the CRS in the case of a cellbelonging to a measurement object (or a neighbor cell list) includingmeasSubframePatternNeigh. In other words, when the measurement subframepattern is received, the UE may assume that corresponding cells arealways in the ON state. Alternatively, an eNodeB may providenotification of the ON/OFF state of the corresponding cells so that theUE performs measurement using the CRS on a restricted subframe indicatedby the measurement subframe pattern if a corresponding cell is ON. Incontrast, if a corresponding cell is OFF, the UE may perform measurementusing a discovery signal by neglecting the measurement subframe pattern,that is, without being restricted by a subframe indicated by themeasurement subframe pattern. If UE has been configured to detect thediscovery signal of a corresponding cell or to perform measurement usinga discovery signal, the UE may neglect a measurement subframe patternand perform discovery signal-based measurement. In this case, the UE mayperform both a CRS-based measurement report and a discovery signal-basedmeasurement report.

If discovery signal-based RSRP/RSRQ measurement is to be performed,whether or not to apply a measurement subframe pattern may be differentdepending on the type of discovery signal and a transmission subframeregion.

Accordingly, this specification proposes that when UE performs discoverysignal-based RSRP/RSRQ measurement, an eNodeB notifies the UE that theUE has to use which one of the first embodiment and the secondembodiment (i.e., when the UE performs discovery signal-based RSRP/RSRQmeasurement, whether a restricted subframe indicated by a measurementsubframe pattern will be applied or not) through high layer signaling.Specifically, when discovery signal-based RSRP/RSRQ measurement isperformed, whether the first embodiment scheme or the second embodimentscheme will be used may be configured for each frequency or eachmeasurement object. Such a configuration may be performed in such amanner that each of or both measSubframePatternPCell andmeasSubframePatternNeigh are applied to a case where CRS-based RSRP/RSRQmeasurement is performed or a case where RSRP/RSRQ measurement based onboth a discovery signal and a CRS is performed. Specifically, whetherspecific measSubframePatternNeigh is applied to a case where onlyCRS-based RSRP/RSRQ measurement is performed or a case where RSRP/RSRQmeasurement based on both a discovery signal and a CRS is performed maybe configured for each measurement object. SincemeasSubframePatternNeigh is present in each measurement object,information indicating that specific measSubframePatternNeigh is alsoapplied to a case where RSRP/RSRQ measurement based on both a discoverysignal and a CRS is performed may be included in the measurement object.In this case, there is an advantage in that flexibility can be providedto a network.

III. Improvement of RSSI Definition

As described above, in accordance with the existing definition, an RSSIis calculated based on a measurement result on a specific OFDM symbolincluding a CRS. However, if such existing definition is directlyapplied to a discovery signal, the following problems may occur. It isassumed that a neighbor small cell sends a discovery signal in the OFFstate. In this case, since no data is sent on a downlink subframe, thereare problems in that a measured RSSI is inaccurate and the calculationof RSRQ is inaccurate.

As a solution for the problems, the definition of an RSSI may beimproved as follows.

In the first example (Option A) of the improved scheme, improvement maybe performed so that RSSI measurement is performed on all the OFDMsymbols.

In the second example (Option B) of the improved scheme, improvement maybe performed so that RSSI measurement is performed on a symbol in whicha discovery signal is not transmitted (i.e., a non-DS-transmittingsymbol).

In the third example (Option C) of the improved scheme, improvement maybe performed so that RSSI measurement is performed on a subframe inwhich a discovery signal is not transmitted (i.e., a non-DS-transmittingsubframe).

The first example (Option A) is described below in detail. In order toaccurately reflect interference indicated by an RSSI, the RSSI may bemeasured on all the OFDM symbols on a subframe in which a discoverysignal is detected. This method may be effective when a measurementsubframe pattern is applied. The reason for this is that the measurementsubframe pattern is determined by taking into consideration an ABS.

The second example (Option B) is described below in detail. The reasonwhy RSSI measurement is performed on a symbol in which a discoverysignal is not transmitted (i.e., a non-DS-transmitting symbol) may bedivided in various ways as follows. First, RSSI measurement may beperformed using OFDM symbols in which a discovery signal is nottransmitted (Option B-1). Next, RSSI measurement may be performed usingOFDM symbols that cannot be used for discovery signal transmission(Option B-2). Finally, RSSI measurement may be performed using OFDMsymbols that have been configured by an eNodeB or that have beenpreviously defined (Option B-3).

Hereinafter, a DS-PSS/DS-SSS may be illustrated as being transmittedthrough a new OFDM symbol region not OFDM symbols #5, #6, but is assumedto be transmitted through the OFDM symbols #5, #6 for convenience ofdescription.

The Option B-1 is described in more detail below. UE may perform RSSImeasurement using OFDM symbols in which the discovery signal of a targetcell or transmission point that performs RSSI measurement is nottransmitted. For example, if the discovery signal of a specific cell ortransmission point includes a DS-PSS/DS-SSS (i.e., the discovery signalis transmitted on the OFDM symbols #5, #6) or includes a DS-CRS (i.e.,the discovery signal is transmitted on OFDM symbols #0, #4, #7, #11), UEmay perform RSSI measurement using OFDM symbols #1, #2, #3, #8, #9, #10,#12, #13 in which a discovery signal is not transmitted.

For another example, if the discovery signal of a specific cell ortransmission point includes a DS-PSS/DS-SSS (i.e., the discovery signalis transmitted on the OFDM symbols #5, #6) or includes a DS-CSI-RS(i.e., the discovery signal is transmitted on OFDM symbols #9, #10), UEmay perform RSSI measurement on OFDM symbols #0, #1, #2, #3, #4, #7, #8,#11, #12, #13 in which the discovery signal is not transmitted.

Furthermore, if a discovery signal includes aDS-PSS/DS-SSS/DS-CRS/DS-CSI-RS, UE performs RSSI measurement on an OFDMsymbol region in which RSs are not transmitted by a cell or transmissionpoint that performs measurement. Alternatively, UE may perform RSSImeasurement on an OFDM symbol region in which a DS-CSI-RS is nottransmitted by taking into consideration a sharing cell ID operationenvironment.

In such a scheme, UE may perform RSSI measurement on OFDM symbols inwhich an RS (DS-CRS and/or DS-CSI-RS) for performing measurement in thediscovery signal of a target cell or transmission point performing RSSImeasurement is not transmitted.

The Option B-2 is described in more detail below. First, if a discoverysignal includes a DS-PSS/DS-SSS/DS-CR, UE may perform RSSI measurementon the remaining symbol region other than candidate OFDM symbol regionsin which a DS-PSS/DS-SSS/DS-CRS may be transmitted. In such a scheme, UEmay perform RSSI measurement on the remaining symbol region exceptcandidate OFDM symbols in which an RS (DS-CRS and/or DS-CSI-RS) forperforming measurement may be transmitted. That is, in this case, asymbol region that belongs to the regions of OFDM symbol for RSSImeasurement to be described in the following examples and in which an RSused in measurement is not transmitted is excluded. An OFDM symbolregion in which only a DS-PSS, DS-SSS is transmitted also excludes anOFDM symbol region in which a DS-PSS, DS-SSS is transmitted from RSSImeasurement. Accordingly, a description is given below based on FDD, forexample. An OFDM symbol region in which a DS-CRS is transmitted maycorrespond to OFDM symbols #0, #4, #7, #11 in a specific cell and maycorrespond to OFDM symbols #0, #1, #4, #5, #7, #8, #11, #12 in anothercell. In this case, UE may perform RSSI measurement on the remainingOFDM symbol region other than the OFDM symbols #0, #1, #4, #5, #6, #7,#8, #11, #12. An OFDM symbol region for such RSSI measurement may bedifferent depending on the number of antenna ports in which a DS-CRS istransmitted. The DS-CRS is transmitted on the OFDM symbols #0, #4, #7,#11 through antenna ports 0, 1, but is transmitted on the OFDM symbols#0, #1, #4, #5, #7, #8, #11, #12 through antenna ports 2, 3.Accordingly, an OFDM symbol region in which RSSI measurement isperformed may be different depending on the number of antenna ports thatmay be included in a DS-CRS. That is, if the number of antenna portsthat may be included in a DS-CRS is 1 and/or 2, UE may perform RSSImeasurement on the remaining OFDM symbol region other than the OFDMsymbols #0, #4, #5, #6, #7, #11. If the number of antenna ports that maybe included in a DS-CRS is 1˜4, UE may perform RSSI measurement on theremaining OFDM symbol region other than the OFDM symbols #0, #1, #4, #5,#6, #7, #8, #11, #12. The number of antenna ports that may be includedin a DS-CRS may be different according to each frequency. In this case,UE needs to perform RSSI measurement on another OFDM symbol region usingthe number of antenna ports of a DS-CRS that has been configured foreach frequency.

If a discovery signal includes a DS-PSS/DS-SSS/DS-CSI-RS, UE may performRSSI measurement on the remaining symbol region other than candidateOFDM symbol regions in which the DS-PSS/DS-SSS/DS-CSI-RS may betransmitted. For example, a description is given below based on FDD.Assuming that a DS-CSI-RS may have all CSI-RS RE configurations, theDS-CSI-RS may be transmitted on OFDM symbols #5, #6, #9, #10, #12, #13.In this case, UE may perform RSSI measurement on the remaining OFDMsymbol region other than the OFDM symbols #5, #6, #9, #10, #12, #13. Inparticular, assuming that data is muted on an RE region in which theDS-CSI-RS of another cell or transmission point may be transmittedthrough a ZP (zero-power) CSI-RS configuration, UE may perform RSSImeasurement on an OFDM symbol region that has not been configured as theZP CSI-RS. Since an OFDM symbol region in which a DS-CSI-RS may betransmitted is different in FDD and TDD, an OFDM symbol region in whichUE performs RSSI measurement may be different depending on the FDD/TDDtype of a frequency in which the UE performs measurement.

If a discovery signal includes a DS-PSS/DS-SSS/DS-CRS/DS-CSI-RS, UEperforms RSSI measurement on an OFDM symbol region in which each RS maynot be transmitted (in a frequency in which measurement is performed).Alternatively, UE may perform RSSI measurement on an OFDM symbol regionin which a DS-CSI-RS cannot be transmitted by taking into considerationthe sharing cell ID operation environment. If the Option B-2 is used,all the discovery signals that may be included in neighbor cells can beprevented from influencing RSSI measurement.

In the Option B-3, UE performs RSSI measurement on an OFDM symbol regionthat has been previously defined or an OFDM symbol region configured byan eNodeB. If an OFDM symbol region for RSSI measurement has beenconfigured by an eNodeB, the OFDM symbol region for such RSSImeasurement may be configured for each frequency.

In this case, if UE performs RSSI measurement on a previously definedOFDM symbol region, an OFDM symbol region for RSSI measurement may bedetermined as OFDM symbols #0, #1, #2, #3 within a first slot. Such aregion can prevent an RSSI value from almost becoming a 0 (zero) valuebecause it is a location that excludes locations, such as the locationof a DS-PSS, DS-SSS, and DS-CSI-RS, and that includes some of a DS-CRS.Furthermore, if a DS-CRS is not included in a discovery signal, a CRStransmitted in the ON state may be included in RSSI measurement so thata more accurate RSSI value is measured. Alternatively, an OFDM symbolregion for RSSI measurement may be determined as the OFDM symbols #1,#2, #3 of a first slot. Such a region is a location other thanlocations, such as a DS-PSS, a DS-SSS, a DS-CSI-RS, and a DS-CRS.

If the sharing cell ID operation environment is assumed, a PCID may bedetected using a DS-PSS/DS-SSS (/DS-CRS), and the ID (transmission pointID) of a transmission port may be detected as a DS-CSI-RS. In this case,the transmission point ID may mean the RE configuration index orscrambling index (or an index configured through an RE configurationindex and scrambling index) of the DS-CSI-RS. In this case, RSRP/RSRQmeasurement for the cell (or cluster) of a specific PCID may beperformed using a DS-CRS, and the RSRP/RSRQ measurement of eachtransmission point (i.e., each transmission point within a cluster)using the same PCID may be performed using the DS-CSI-RS. In this case,an OFDM symbol region in which the RSSI measurement of a cell (cluster)for a specific PCID is performed using the DS-CRS may be different froman OFDM symbol region in which the RSSI measurement of a transmissionpoint using the DS-CSI-RS. For example, assuming that the OFDM symbolregion in which the RSSI measurement of a cell (or cluster) for aspecific PCID is performed using the DS-CRS includes part of the entireOFDM symbol region, if RSSI measurement is performed on a transmissionpoint using the DS-CSI-RS, an OFDM symbol region in which the DS-CRS istransmitted may be excluded from a symbol region for measurement. Forexample, when the RSRP/RSRQ of a cell (or cluster) is measured using aDS-CRS, an OFDM symbol region in which RSSI measurement is performed maybe identical with the OFDM symbols #0, #1, #2, #3 of a first slot.However, if the RSRP/RSRQ of a transmission point is measured using aDS-CSI-RS, an OFDM symbol region in which RSSI measurement is performedmay be identical with the OFDM symbols #1, #2, #3 of a first slot otherthan an OFDM symbol region in which a DS-CRS is transmitted.

The third example (Option C) (i.e., RSSI measurement on a subframe inwhich a discovery signal is not transmitted) is described in detailbelow. In the second example (Option B), UE measures the RSSI of an OFDMsymbol (i.e., a non-DS-transmitted OFDM symbol) in which a discoverysignal is not transmitted. In this case, however, if the number of OFDMsymbols occupied by a discovery signal is great, the number of OFDMsymbols in which RSSI measurement may be performed may not besufficient. In order to solve such a problem, in the third example(Option C), UE may perform RSSI measurement on a subframe in which adiscovery signal is not transmitted. In this case, an eNodeB may notifythe UE of the location of a subframe in which RSSI measurement will beperformed. However, in order to reduce signaling overhead or withconsideration taken of that a configuration is not required, UE may bemade to implicitly know the location of a subframe in which RSSImeasurement is to be performed. UE may receive a DMTC from an eNodeB.Such a DMTC may include a cycle and offset value and may also include aduration value. Accordingly, in this case, there is proposed that RSSImeasurement be performed in a subframe next to a subframe in which PCID(transmission point ID) of a cell or transmission point on which UE willperform measurement. Alternatively, if a cycle and offset value areincluded in a DMTC, UE may perform RSSI measurement on a subframe (i.e.,(n−1)-th subframe) that is one prior to the location of a subframe(i.e., an n-th subframe) indicated by a corresponding configuration or anext subframe (i.e., an (n+1)-th subframe). Alternatively, UE mayperform RSSI measurement on a subframe including a subframe (i.e.,(n−1)-th subframe) that is one prior to the location of a subframe(i.e., an n-th subframe) indicated by a corresponding configuration or anext subframe (i.e., an (n+1)-th subframe). Alternatively, UE mayperform RSSI measurement on a subframe other than the location of asubframe indicated by a corresponding configuration. Such options may beuseful in the case where a cell or transmission points sends discoverysignals in the same subframe and the discovery signal includes a singlesubframe.

As described above, a DMTC may include a cycle and an offset value andmay additionally include a duration value. In this case, when UEreceives a DMTC from an eNodeB, the UE may perform RSSI measurement on asubframe other than the received timing duration. In this case, the UEmay perform RSSI measurement on a next subframe after the timingduration indicated within a configuration is ended.

In another method of performing, by UE, RSSI measurement in a subframein which a discovery signal is not transmitted, the UE may perform RSSImeasurement on a subframe during for specific duration. In this case, anactual discovery signal may be received only on some subframes of thecorresponding duration. For example, in such a measurement method, UEmay perform RSSI measurement on each subframe for specific duration bytaking into consideration the location of a subframe in which adiscovery signal is actually transmitted, may average RSSI values, andmay report a result of the RSSI measurement. In this case, the value ofthe corresponding duration may be included in a DMTC. Alternatively, thevalue of the corresponding duration may be a duration value for RSSImeasurement that has been configured by an eNodeB separately from aduration value at which a discovery signal is expected to betransmitted. Alternatively, a separate timing configuration (e.g., acycle, an offset and/or duration) for RSSI measurement may be configuredfor UE, and the UE may perform RSSI measurement on subframes during thecorresponding duration, may average RSSI values measured for thecorresponding duration, and may report a result of the average.

An eNodeB may configure that RSSI measurement has to be performed usingwhich one of the Option A scheme and the Option B scheme for UE.Specifically, such a configuration may be performed for each frequency.That is, if UE has performed RSSI measurement using a discovery signal,an eNodeB may configure, for UE, that the UE has to perform RSSImeasurement on an OFDM symbol region before a subframe in which adiscovery signal is received (as described above in the Option A) orperform RSSI measurement on a (or some) OFDM symbol region in which adiscovery signal is not received unable to be received (as describedabove in the Option B).

If a high layer has instructed that RSSI measurement be performed on allOFDM symbols, UE may perform RSSI measurement on all the symbols withina subframe for measurement. In this case, an eNodeB may configure thatdiscovery signal-based RSSI measurement must be performed using theOption B scheme for UE (for a specific frequency) and also configurethat RSSI measurement must be performed on all OFDM symbols for the UE.In this case, the UE may perform RSSI measurement on a cell ortransmission point on which discovery signal-based measurement needs tobe performed through the Option B scheme and perform RSSI measurement onall OFDM symbols on a cell or transmission point on which measurementneeds to be performed according to an existing scheme through a highlayer signal. This may be generalized as follows. UE may performmeasurement on a cell or transmission point on which measurement isperformed using an existing scheme using the existing scheme and performmeasurement on a cell or transmission point on which discoverysignal-based measurement is performed using a configuration related to adiscovery signal.

Alternatively, UE may neglect high layer signaling instructing that RSSImeasurement be performed on all OFDM symbols and perform measurementthrough the Option B scheme. This may be generalized as follows. UE maycomply with a discovery signal configuration for all cells ortransmission points, assuming that a discovery signal-relatedconfiguration is the top priority with respect to a cell or transmissionpoint on which measurement is performed using an existing scheme and acell or transmission point on which discovery signal-based measurementis performed.

If a high layer instructs that RSSI measurement be performed on all OFDMsymbols, the instruction of such a high layer may also be applied toRSSI measurement using a discovery signal.

In this case, such a high layer signal may have priority when UEconfigures an OFDM symbol region for RSSI measurement. Specifically, UEbasically assumes RSSI measurement on a (or some) OFDM symbol region inwhich a discovery signal is not received or cannot be received (asdescribed above in the Option B). If a high layer signal instructingthat RSSI measurement be performed using the entire OFDM symbol regionis received from an eNodeB, the UE may perform RSSI measurement usingthe entire OFDM symbol region in a subframe in which a discovery signalis transmitted (as described above in the Option AP).

Alternatively, in order to prevent an RSSI from being over estimated dueto the discovery signal of a cell in the OFF state, UE may neglect ahigher layer signal although the higher layer signal instructing thatRSSI measurement be performed on all OFDM symbol. For example, UE mayperform RSSI measurement in a (or some) OFDM symbol region in which adiscovery signal is not received or cannot be received (as in the OptionB), may neglect a high layer signal instructing that RSSI measurement beperformed in the entire OFDM symbol region although the UE receives thehigh layer signal from an eNodeB (as in the Option B), and may performRSSI measurement in a (or some) OFDM symbol region in which a discoverysignal is not received or cannot be received (as in the Option B).

If UE receives a measurement subframe pattern instructing thatmeasurement be performed on a restricted subframe from an eNodeB, the UEperforms RSSI measurement on all OFDM symbols within the restrictedsubframe.

In this case, specifically, the UE basically assumes that RSSImeasurement will be performed in an (or some) OFDM symbol region inwhich a discovery signal is not received or cannot be received (as inthe Option B). If the UE receives a measurement subframe pattern from aneNodeB, the UE may perform RSSI measurement in the entire OFDM symbolregion within a subframe that belongs to restricted subframes and inwhich the discovery signal is received.

Alternatively, in order to prevent an RSSI from being over estimated dueto the discovery signal of a cell in the OFF state, if UE receives ameasurement subframe pattern from an eNodeB, it may perform RSSImeasurement in a (or some) OFDM symbol region in which a discoverysignal is not received or cannot be received in a restricted subframe(as in the Option B).

Alternatively, in order to prevent an RSSI from being over estimated dueto the discovery signal of a cell in the OFF state, if UE receives ameasurement subframe pattern from an eNodeB, it may neglect theconfiguration of the measurement subframe pattern.

If a discovery signal includes a PSS/SSS/CRS/CSI-RS (specifically, if adiscovery signal includes a DS-CSI-RS), UE may perform RSSI measurementin a (or some) OFDM symbol region in which a discovery signal is notreceived or cannot be received although it receives an instruction thatRSSI measurement be performed in the entire OFDM symbol region (as inthe Option B). In contrast, if a discovery signal includes a PSS/SSS/CRS(i.e., if a discovery signal does not include a DS-CSI-RS), if UEreceives an instruction that RSSI measurement be performed in the entireOFDM symbol region, it may perform RSSI measurement on all OFDM symbolsin response to the instruction. If the corresponding instruction is notreceived, the UE may perform RSSI measurement only on OFDM symbols inwhich a CRS (or DS-CRS) is received as in the existing RSSI measurement.If a DS-CSI-RS has been configured, UE may comply with a new RSSImeasurement method. If a DS-CSI-RS has not been configured, the UEperforms RSSI measurement according to the existing scheme.

Specifically, if RSSI measurement is performed on OFDM symbols in whicha DS-PSS/DS-SSS is received, RSSI measurement may not be performed in anOFDM symbol region or the entire symbol region in which a DS-PSS/DS-SSSis received in a bandwidth (e.g., the center 6 PRBs) region in which theDS-PSS/DS-SSS is received. Alternatively, specifically, when RSSImeasurement is performed in a subframe in which a discovery signal isreceived, RSSI measurement may not be performed in a region other thancenter 6 PRBs. In this case, a discovery signal can be prevented frombeing deviated (i.e., a biased RSSI measurement result can be preventedfrom appearing) because the amount of resources occupied by a discoverysignal within the center 6 PRBs is great. Alternatively, in this case,when RSSI measurement is performed only in a symbol region in which adiscovery signal is not received, RSSI measurement may be performed ifRSSI measurement symbol resources are not sufficient within the center 6PRBs because an OFDM symbol region in which a discovery signal isreceived is sufficient.

IV. CSI/CQI Measurement

First, there may be several schemes for measuring RSSI as describedabove. As the RSSI measurement scheme is diversified as described above,a CQI may also be influenced. Accordingly, the following is proposed.

For CQI interference measurement, UE may not measure interference on anOFDM symbol including a discovery signal (or on an OFDM symbol includinga discovery signal in a given subframe). For example, if a CRS isincluded in a discovery signal, the discovery signal may be receiveddepending on a discovery signal timing configuration, but interferencemay not be measured on an OFDM symbol including the CRS.

For CQI interference measurement, if a discovery signal-based RSSI ismeasured on a subframe different from a subframe in which the discoverysignal is received, when CQI interference is calculated, the subframe inwhich the discovery signal is received may need to be excluded.Accordingly, UE does not use a subframe in which DS is received withrespect to interference measurement for CQI measurement. In other words,CQI interference measurement complies with RSSI definition.

For aperiodic CQI measurement, if a downlink subframe indicated by anaperiodic CSI request is a subframe in which a discovery signal isreceived according to the configuration of the discovery signal, UE doesnot consider the subframe to a valid downlink subframe. Alternatively,such a subframe may be excluded by network scheduling. Accordingly, ifan aperiodic CSI request is present on a subframe, UE will stillconsider a subframe in which a discovery signal is received to be avalid subframe.

If a downlink subframe indicated by an aperiodic CSI request is includedin a subframe indicated by a DMTC with respect to an aperiodic CQIrequest, the corresponding subframe may be considered to be not a validdownlink subframe. Alternatively, such a subframe may be excluded bynetwork scheduling. Accordingly, if an aperiodic CSI request is presenton a subframe, UE may still consider a subframe in which a discoverysignal is received to be a valid subframe.

V. RSSI Measurement Subframe

If timing is not synchronized between small cells in a small cellenvironment, although the cells have the same discovery signaltransmission timing, timing at which each cell actually sends adiscovery signal may be different. This is described with reference toFIG. 20.

FIG. 20 illustrates another example in which transmission timing ofdiscovery signals is different between cells.

As may be seen with reference to FIG. 20, although all the discoverysignals of a cell#1, a cell#2, . . . , a cell#5 are identicallytransmitted in subframes #n, #n+1, #n+4, if subframe timing is the samebetween the cells, timing at which each cell sends a discovery signalmay be different.

In this state, assuming that the serving cell of specific UE is thecell#1, if the UE tries to perform discovery signal-based RSSI (orcalled DSSI) measurement, there may be a problem in that a measured DSSIvalue may be different depending on the configuration of the location ofa subframe on which the measurement is performed because timingsynchronization is not the same. Accordingly, methods of measuring asearch DSSI in the following subframe duration in order to solve such aproblem are hereinafter proposed.

In a first scheme, UE may check subframe duration in which neighborcells sending discovery signals send the discovery signals in commonaccording to a DMTC in order to measure DSSI and measure the DSSI usingonly the corresponding subframe duration. For example, as illustrated inFIG. 20, if UE is aware of the discovery signal transmission timing ofthe cell#1, that is, a serving cell, and the cell#2, the cell#3, thecell#4, and the cell#5, that is, neighbor cells, the UE may use only thesubframes #n+1, #n+2, #n+3, that is, a subframe region in which thecell#1, . . . , the cell#5 send discovery signals in common for DSSImeasurement. Furthermore, in the case of TDD, since such a problem isnot present, such a configuration may be said to be limited to FDD.Furthermore, in order to align subframes in which discovery signal-basedRSRP and RSSI are measured, it may be assumed that discoverysignal-based RSRP measurement is performed in duration used in themeasurement in DSSI.

In a second scheme, an eNodeB may configure the locations of subframesto be used for DSSI measurement for UE. For example, the location of asubframe for DSSI measurement may be included in a DMTC and configured.In order to represent the location of the subframe for DSSI measurement,the following values may be set.

-   -   An offset value from the start point of DMTC duration for        indicating the start points of subframes for DSSI measurement.    -   The duration value of a subframe for DSSI measurement.

In this case, UE may use only “subframes for DSSI measurement”configured within DMTC duration for DSSI measurement. Furthermore, inorder to align subframes in which discovery signal-based RSRP and RSSIare measured, it may be assumed that discovery signal-based RSRPmeasurement is performed only in duration used for DSSI measurement.

In a third scheme, in order to perform DSSI measurement, UE may use theentire DMTC duration for the DSSI measurement regardless of the locationof a subframe in which a discovery signal is actually transmitted. Forexample, although the subframes #n˜#n+4 are configured according to aDMTC and the discovery signal of a serving cell is transmitted only inthe subframes #n˜#n+2, UE may measure DSSI only in the subframes#n˜#n+4, that is, DMTC duration. Furthermore, in order to alignsubframes in which discovery signal-based RSRP and RSSI are measured, itmay be assumed that discovery signal-based RSRP measurement is performedonly in duration used for DSSI measurement.

In a fourth scheme, UE assumes that a discovery signal is controlled bya network so that it is received in duration indicated by a DMTC.However, in this case, if cells are not synchronized, there may be adifference of a maximum of one subframe between the cells because asubframe boundary may be different. Accordingly, in this case, there maybe a change of interference at the start point and end point of the DMTC(e.g., the discovery signals of only some cells may be transmitted).Accordingly, if a DMTC is received, UE may assume that DSSI measurementcan be performed only in duration other than former/latter 1 msec. Sucha configuration may be applied regardless of the synchronization of anetwork. Specifically, 1 msec before/after DMTC duration may be assumedto be not used for DSSI measurement. Furthermore, in the case of TDD,such a configuration may be said to be limited to only FDD because sucha problem is not present. Furthermore, in order to align subframes inwhich discovery signal-based RSRP and RSSI are measured, discoverysignal-based RSRP measurement may be assumed to be performed in subframeduration used in DSSI measurement according to the proposed method. Thatis, if DMTC duration is 5 msec, discovery signal-based RSRP/RSRQmeasurement may be performed only in middle 3 msec.

The embodiments of the present invention described so far may beimplemented through various means. For example, the embodiments of thepresent invention may be implemented by hardware, firmware, software, ora combination of them. This is described with reference to FIG. 21.

FIG. 21 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

A BS 200 includes a processor 201, a memory 202, and an RF (radiofrequency) unit 203. The memory 202 coupled to the processor 201 storesa variety of information for driving the processor 201. The RF unit 203coupled to the processor 201 transmits and/or receives a radio signal.The processor 201 implements the proposed functions, procedure, and/ormethods. In the aforementioned embodiment, an operation of the BS may beimplemented by the processor 201.

The UE 100 includes a processor 101, a memory 102, and an RF unit 103.The memory 102 coupled to the processor 101 stores a variety ofinformation for driving the processor 101. The RF unit 103 coupled tothe processor 101 transmits and/or receives a radio signal. Theprocessor 101 implements the proposed functions, procedure, and/ormethods. In the aforementioned embodiment, an operation of the wirelessdevice may be implemented by the processor 101.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The RF unit may include a base-bandcircuit for processing a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory and may be performed by the processor. The memory may be locatedinside or outside the processor, and may be coupled to the processor byusing various well-known means.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.Further, it will be understood by those ordinary skilled in the art thatthe steps of the flowcharts are not exclusive. Rather, another step maybe included therein or one or more steps may be deleted within the scopeof the present invention.

What is claimed is:
 1. A method for performing measurements, the methodperformed by a user equipment (UE) and comprising: applying both of ameasurement subframe pattern for a neighbor cell and a measurementtiming configuration for a discovery signal; and selecting at least oneor more subframes, to perform the measurement according to the applianceof the measurement subframe pattern and the measurement timingconfiguration.
 2. The method of claim 1, further comprising: receivingthe measurement subframe pattern for the neighbor cell and themeasurement timing configuration for the discovery signal.
 3. The methodof claim 1, wherein the measurement subframe pattern for the neighborcell is different from the measurement timing configuration for thediscovery signal.
 4. The method of claim 1, wherein the measurementtiming configuration is configured per carrier frequency.
 5. The methodof claim 1, further comprising: if the neighbor cell is in thedeactivated state, using the discovery signal rather than acell-specific reference signal (CRS) to perform the measurements.
 6. Themethod of claim 1, wherein the discovery signal is a signal based on atleast one of cell-specific reference signal (CRS), a channel-stateinformation reference signal (CSI-RS), a primary synchronization signal(PSS) and a secondary synchronization signal (SSS).
 7. The method ofclaim 1, wherein if the measurement is for measuring a received signalstrength indicator (RSSI), the measurement is performed on entireorthogonal frequency division multiplexing (OFDM) symbols of a subframe.8. A user equipment (UE) for performing measurements, comprising: atransceiver; and a processor operatively connected to the transceiverand configured to: apply both of a measurement subframe pattern for aneighbor cell and a measurement timing configuration for a discoverysignal; and select at least one or more subframes, to perform themeasurement according to the appliance of the measurement subframepattern and the measurement timing configuration.
 9. The UE of claim 8,wherein the processor is further configured to: receive the measurementsubframe pattern for the neighbor cell and the measurement timingconfiguration for the discovery signal.
 10. The UE of claim 8, whereinthe measurement subframe pattern for the neighbor cell is different fromthe measurement timing configuration for the discovery signal.
 11. TheUE of claim 8, wherein the measurement timing configuration isconfigured per carrier frequency.
 12. The UE of claim 8, wherein theprocessor is further configured to: use the discovery signal rather thana cell-specific reference signal (CRS) to perform the measurements, ifthe neighbor cell is in the deactivated state.
 13. The UE of claim 8,wherein the discovery signal is a signal based on at least one ofcell-specific reference signal (CRS), a channel-state informationreference signal (CSI-RS), a primary synchronization signal (PSS) and asecondary synchronization signal (SSS).
 14. The UE of claim 8, whereinif the measurement is for measuring a received signal strength indicator(RSSI), the measurement is performed on entire orthogonal frequencydivision multiplexing (OFDM) symbols of a subframe.